What do his bowels look like when he does have them?
There could be a number of reasons he is not gaining weight... and it's good to find out now. I could see if he was always smaller and just not gaining quickly, but to really decrease (or gaining weight and length very slowly) is not good- UNLESS... you and your husband are small built people. If so, as long as your son is satisfied and healthy, he should be fine.
Seriously, I would go back to the 2-1/2 hours feedings, if he is sleeping through the night that well. Or you might wake him up a little early then let him take an extra nap in the morning.
My youngest son stopped gaining much weight between 3-4 months, but we found out a month or so later that he also has a genetic disease which can effect digestion (reason for asking about the bowel movements- my sons were very mushy and bulky and often, could easily fill a diaper).