D.H.
If the doctor is ordering it, can he write is as something the insurance pays for? If not, ask the Dr. office where you can find it much cheaper, maybe through some health care or kids center........they surely have to have someone to help. Try the local Children's hospital as well.
Get on Google and put in Good Start coupons.....and see what happens, you may have to add more or less info to it, but give it a shot..............
Can he eat other things that are high in calories that are healthy for him.......
I'll say a prayer for you.....Good luck and take care.