Thanks all for your words of encouragement!
Madcat - great advice about the warm drink, I will try that.
Esther - he is only going to be doing the 1 club session each week and will do 2 of his usual (30min, usually widths) lessons (in a warmer pool!) alongside, and an additional hour long 1:1 which he loves (but is mainly playing! (diving/turns/under water swimming, picking up bricks, life saving etc..)!! lots of resting and not much hard work! But some lengths!).
The club session was a full-on fast-lengths workout for an hour! Including 6 lengths of butterfly! (He can normally only manage 1! and I thought that was good!!) I think the cold pool doesn't help as DS is super skinny and small (it's even cold for me and I have loads of padding!!) My DS's character also doesn't help as he is VERY competitive and determined. Apparently the coach offered him to sit out some lengths/do an alternative stroke, but he was determined to keep up and chose not to. He was properly shaking though at the end of the training - felt sick, shivering, aching all over (scream - yes! DS also couldn't dress himself or shower and was barely standing at the end!)
I will bring a hot drink, warmer towel, and maybe a onesie! to the next session! And will also talk to him seriously about sitting out some lengths and not pushing himself too hard, especially if the coaches are offering him that.
We'll try a few more and see how it goes!
Thanks for all the reassurance!