D.S.
S.,
What kind of carseat do you have him in? Let me know and I might be able to give you some pointers! Also, how tall is he and how much does he weigh?
Take a look at this link:
http://www.carseat.org/Resources/635.pdf - the first thing on there asks if his harness straps are tight enough - are they?
http://www.carseatsite.com/pinchtest.jpg - if you can pinch any slack on the straps like in this picture then the straps are definitely too loose!
2 1/2 is definitely too young to be using the seatbelt though, a seatbelt is much much easier to unbuckle than the carseat itself! (Car seat technicians recommend keeping a child harnessed until they are at least 4 or 5, and at least 40lbs. After that we think about moving them into a booster, but the fact is, most 4 and 5 year olds aren't mature enough to use a booster, whether high back or backless, properly, every time.)
I know this has got to be really tough for you! Get back to me and we can try and figure out a solution.