I have just spoken with Riding for Disabled and lady was very helpful, but I hadn't realised that sessions with them would be during school time. The school might be ok with this, but possibly not as he is at a mainstream school and his attendance is not great.
what are the alternatives? He has dyspraxia, poor muscle tone and some hearing loss. I think riding will really help develop his core muscle strength, but we would need an understanding instructor as his abilities and needs are different to your average 5 year old.