Your dd is still only 15.6 lbs., 25" at 3 months old so I would keep him in his infant seat until he reaches the max. At 3 mths, personally, I would want to keep him in an infant seat so I could use it as a carrier as well.
What infant seat do you have him in now? What is the weight limit?
If you want to put him in a convertible now because he is at the limit of the seat he is in, than the Britax Marathon 70 or the Boulevard, or the Graco Myride are great choices (all max at 40lbs. and 49" rear facing :yeahthatlove
, and comfy for a 3 mth. old.
Some people will do a Radian for a child that size, but I have one for my 4 yo., and personally, I would not be thrilled about putting a 3mth old in it because it just doesn't look as comfy and supportive to me as a Marathon, Boulevard or a Myride would be.