My son has autism

My son has autism, he is three going on four, his diet is horrible ..for obvious reasons! 

He has no Fruits or veg or any grains ....basically any fibre in his a diet, we give him a vitamin tablet everyday which turns into a juice and he enjoys but he has 0 Fibre in his diet so I'm worried about all lost of health issues that come with zero fibre in his diet

My question is Can I give him a fibre supplement and if so what gram would you recommend.