Did he see the doctor for his 4month shots? The concern isn't so much what he weighs as it is that he is following a proper growth curve. Fourteen pounds does not necessarily mean he is underweight. If he really isn't gaining any weight, I would call the doctor to discuss. My girls are very small and are both charting their own curve Their stats:
Audrey - 5lb8oz at birth. 10lb14oz at 4.5 months = 5th percentile
Evelyn - 6lb1oz at birth. 11lb11oz at 4.5 months = 12th percentile
Although petite, they are growing at a proper rate and are not considered underweight.
Ditto to what sunnybrook said. As long as they are steadily gaining some weight, I wouldn't worry.
H&J are 5 months are as of last week are 13lb12oz and 12lbs12oz. They are petite, but definitely are gaining weight.
They had their 4-month shots at 4-month, 1-week and were 12lbs 1oz (5lbs 0oz at birth) and 12lbs 5oz (5lbs 3oz at birth). Their pedi wasn't concerned at all. I'm not sure how accurate or "good" this website is, but here's one I've found to help with percentiles:
Jake: 5lbs 7oz at birth 12lbs 9oz at 4 months
Addy: 5lbs 5oz at birth 14lbs 13.5oz at 4 months
If the pedi saw him and didn't say anything, I wouldn't worry about it. The concern lies is if he falls off his growth curve. I just googled percentile charts and several popped up.