Hi there,
I have 18mnth old who has been dignosed with mild cp (low muscle tone). She had issues with constipation and I was advised by fellow moms to use miralax. Even my pharmacist suggested it 1 teaspoon a day (mixed in any food or drink) for few days. It takes few days for it to work but it will be softer and easier to go. She just learned to sit on her own and we are doing physical therapy 2x a week which will change to 3-4 x a week. Find out if your state has early intervention program contact them and they will help you.
Blend his food in a blender the same with fruits (they have fiber). We cook soups for my daughter somethihg like brocolli, carrot, piece of meat and potato for 1 hour on medium and blend it make it liquidi. He might not lke it at first but boy it helps with constapation and weight gain. In low muscle case kids need protein from a good piece of meat and extra meals as tehy tend to burn more calories.
Sorry for a long post hope it helps. HAve they done MRI? We are still waiting on it as she was a little young do do it right of way. Thank God she is only beind in mtor skills all other areas she is ahead of her peers.