Has the developer turned over everything to the association? Based on his ownership, it still sounds like it's developer owned and the percentage swings his way. It will say in your docs.

Joel is correct, you need to do your homework first. 
 
Check your state condominium statues, your covenants, and your bylaws for under what conditions the HOA will gain control. 
 
Since the developer has rented for four years I would think that legally it would be considered as sold because they are occupied. 
 
Ultimately you will probably have to bring in an attorney. We had a similar situation in a non-condominium community. We engaged a lawyer, the lawyer sent the developers lawyer a letter stating we were going to take them to court. Bam, the developer gave up control. 
 
In our condominium community where we currently reside, the developer failed to pay assessments to the HOA once the minimum number of occupied units were sold, as per state law. The HOA sued and one. 
 
Go get them!

Your CCRs (declaration) will outline the process for him to follow. It's usually when a certain % of the units have been sold. Since he still owns 11 of the 24 units I doubt that the time has come yet. in the meantime be glad that he is at least paying the shortfall each month. This will enable you to keep from increasing the HOA assessments while he is still in control. You say you are the Pres of the BOD. That, in itself, is a bit unusual since he is still in control. Does he hold a seat on the board? Are all the other board members unit owners? It appears to me this developer is honest and not trying to be in control which he has a right to be as long as he is the declarant.

With regard to my previous comment; our HOA "won", not "one". 
 
With regard to Mary's comment; the developer is making money by renting. The residents of the rentals are using HOA common areas and other services. The developer needs to pay 100% of the assessments for those rentals. Were those condos vacant, then I would agree.

There is usually a turnover by percentage of units sold or a certain number of years. In our case, it was a 3-year time frame, then developer was supposed to turnover to HOA. 
 
It sounds as if developer wrote the by-laws in his favor. Has anyone seen the 'books'? How do you know there is a shortfall? Is that what he is telling you? Call me crazy but I'd want to know for certain what is happening financially. It's your property involved and your money. 
 
Lastly, my developer kept control of the 'books' until we stopped providing assessments to him and I took control of the HOA. There were problems with the building, he had left town, no lawn care was being done, etc., etc. No surprise to find that he didn't turn over the 'reserves' that I and my neighbor had been paying into for 2 years.  
 
My advice, assume nothing.

Ron, 
 
 
 
The BOD cannot dictate to the developer what they think he should do. According to what the OP wrote, he has a choice of either paying assn.fees or to make up the shortfall each month. He chose the latter. Unless it is specified in the CCRs or bylaws that he must pay assn fees if his units are rented, then he doesn't have to.

Do your self a favor, get a good lawyer, you will save money in the end