Ok, it seems you don't understand the laws and the immigration topic at all.
So, in order to make it clear:
You can do a family reunion from
* Belgium (temporary resident permit - orange card) - you can live in Belgium together during the process
* Outside Belgium (visa D - long stay) - you're far away from each other during the process
with different conditions that can vary depending on the context (nationalities, permanent resident, temporary...)
Everything that I will say in the next explanation is based on the fact that you're Belgian AND living in Belgium.
Cohabitation:
Once he arrives in Belgium, you must immediately go to the commune to declare its arrival + start the cohabitation process. Don't wait for the end of its Schengen Visa, do it ASAP! The sooner, the best!
He'll need its passport and a certificate confirming that he's unmarried (legalized/apostile from the foreign ministry in Turkey + sworn translation in FR/NL/DE/EN + legalized by the embassy/consulate of Belgium in Turkey).
They also may request its certificate of birth / criminal record (legalized/apostile too).
You'll have to pay +/- 20€ at the commune (admin + police visit).
Within 2 weeks (officially, it really depends on your commune), police will come into your accommodation to verify that he's living with you at your address. (They might want to enter and verify its clothes are inside your wardrobe, toothbrush, ...)
If the police check is positive, you'll both be invited to go back to the commune few days later to sign the cohabitation in front of the mayor.
The cohabitation is done.
Meaning:
You both stay officially single (or widow/divorced depending on your case). Your civil status won't change with a legal cohabitation.
BUT your population registry will be modified and you'll be linked to your boyfriend.
==> your family composition will be changed ==> Your boyfriend will be officially a member of your family.
Family Reunion:
Now, being a family member, he's eligible to request a family reunion, and so, a residence permit.
Note: I say he's eligible to request a family reunion, NOT that he will receive it. Immigration will decide if you both respect the laws and the resident permit can be granted.
So you'll request a family reunion at your commune, they will give him an annex 19ter – AR 08/10/1981 (don't ever lose that paper until he gets its F card!).
They'll tell you which papers must be included into the file to send to the immigration in Brussels.
Basically:
* 3 pictures
* proof of adequate accommodation (my rent / certificate of owning)
* certificate from your mutuelle/ziekenfond that you're BOTH affiliated (you need to register him and show them a certificate of family composition + annex 19ter)
* payslip + work contract + financial guarantor + ...
* proof of payment of the royalty fee to the immigration (204€) (statement from your bank)
* proof that you have lived together one year together before
OR proof that you've a child together (certificate of birth)
OR proof that you know each other since 2 years and you keep in touch together (prefer daily)
+ write a letter to explain your relationship, when/where did you meet, how do you communicate since DAY 1, which language, different/same religion, trip together, visas/immigration stamps, pictures, phone call history, chat, ...) Explain your relationship, why a cohabitation together, why not a marriage, ... Defend yourselves!
* ... (it depends on the commune, most probably a medical certificate too)
You have 3 months to give the file to your commune.
As you've already been controlled by the police few days before, in order to declare the cohabitation, the police shouldn't check you again (that fast), but it really depends on the commune you're living...
If the check is positive / no need, your boyfriend will receive a temporary ID card (known as the "orange card"), valid for the next 6 months, extendable until the immigration decide on his case.
He's NOT allowed to travel inside Schengen Area with that temporary resident permit (only if its Schengen Visa is still valid), he can only stay in Belgium.
He's allowed to work as an employee without the need of a work permit (self-employed under strict conditions)
If he's leaving Belgium for any reason, immigration will consider he doesn't want to come in Belgium in order to do a family reunion, and so he'll be denied (unless force majeure (family death, ...) + declared at the commune + request a visa return)
Immigration will give an answer withing 6 months.
If no news after 6 months (date written in the annex 19ter), he'll receive its Family Member of a European citizen ID card (F card), with a validity of 5 years.
That F card can be canceled easily by the immigration within the first 3 years, in case you don't live together anymore / no more working / ...)
Note: the annex 19ter denied any official order from the immigration to leave the country within 30 days, that your boyfriend might have received if its Schengen Visa has expired during the process.
Administration is SLOW, (police check, time to be declared a family member, ...).
So do the math and the calculation! Check if you have enough time to be able to receive the annex 19ter in time or he'll be thrown away from Belgium and won't be able to come back before 2 or 3 years!!!
If it's not doable, get him back outside the Schengen Area before its visa expire and ask a new visa C in few months (maximum of 90 days of stay for a period of 180 days inside the Schengen Area) to do the full process correctly.