J.S.
Has the suggestions from your doctor worked? I went through some issues with my daughter and had to add mineral oil to her bottle. She finally outgrew it at age 3. It is not safe to go without a BM for more that 5-6 days so if what you've given him isnt working, then see another doctor. Up his water intake if you can and add some natural fiber (very little) to his fluids. Maybe that will help. Try a natural route so his body doesnt get lazy and rely on help. (which will cause issues later). Hope this helps!
J.