I think that might mean he has an immature digestive system. That would mean that his body cannot yet digest/process to many solids. I would ask your Dr. to explain though as I cannot say I am certain.
As Margy said, I would go slow. Don't introduce too many things at once..that will help you figure out if there is anything specific he is reacting too. I would also make sure that you are keeping to softer, easier to digest things for now. As mentioned above, pasta is a good one as well as veggies steamed quite soft and cut up small.
Once again, I do suggest you clarify with your Dr. as I could be way off here. But that is what I think he might be saying.
I dont know what he means did you ask him what he means by weak .? Do you let him feed himself, he is old enough for finger foods, put them down in front of him and let him eat in his own time, a lot of children as they get older do not like being spoon fed .Put Cheerios in front of him fruits cut up, fingers of toast spread with a little jelly and if hes okay with it, peanut butter., egg, beans, peas, bites of chicken, little bitty florets of broccoli, pasta, lot of nice shapes out there kiddies love, rice.Make no big issue of it, let him see you eating and let him get on with it,give him plenty of time children do not like being rushed .Good luck