Taking hours to eat

My son is 24. He is 6ft and weighs 9st.12. He has no speech and is unable to communicate any needs or feelings.

He is taking hours to eat the smallest morsel of food,. He holds food and drink in his mouth and chews and chews, swallowing the tiniest amounts at a time.

He has had numerous physical health checks and they can find nothing wrong with him.

He is having build up shakes, prescribed by a dietician ( who was useless) and they take him over an hour to drink.

Has anyone else experienced this?

 It is so hard to ensure he has enough calories and it is making his already restricted life even more restricted.

Many Thanks for taking the time to read this