This sounds like completely normal weight gain to me. When I got my Emma at 7 months old, she was 2.9 lbs. Now at 1 yr and almost 4 months, she weighs almost 4 lbs
She has been at this steady weight for several months now.
If he looks healthy and doesn't look overweight, then I'm sure he's fine
But if you're having trouble feeling his ribs, then you may need to cut back on portions a little bit.