Are you feeding him or are his parents? Unless you actually live with the child 24/7 it would be hard for your to tell how much he actually eats...
I am a firm believe in "they will eat what you offer" if you offer it often enough and they are hungry. He may be drinking to much milk or formula (if he is still on it) to actually be hungry for table foods. In this case it would be on his primary caregiver to reduce the amount of milk he gets in favor of food.
My DD didn't hit 20 lbs until she was 18 months and while she has always been long and lean, she was never underweight, so if he he 25 lbs it sounds like he is doing fine weight-wise.
I think if his mom is concerned she should talk to his pediatrician about it.
Good luck.