Also his length may be growing so fast he's burning through calories and not 'gaining' as much.
Happened to a friend of mine who's child maintained the same weight for 2 months. He was just growing in length so quickly he couldn't keep the weight.
If you are really concerned you can get specialty formula that is for preemies or low weight children (sold behind pharmacy counters) and give him that.
But is wait until after all the bloodwork.