When I got my old ID he was in full work hunting twice a week. Not an ounce of fat on him, he was fit and lean. Probably weighed the same as your boy. The groom used to feed the hunters barley rings if they were too poor.
What an awful situation. If you do a Danilon trial, cover him with a PPI, eg omeprazole, to avoid stomach irritation. Is there any history of injuries whilst in training? Maybe a bad fall?
He’ll be fine. Due to circumstances I had to let two of mine winter out. One wb and one tb type, used to being cosseted and stabled. They grew amazingly thick coats and didn’t need rugged. I’ve lost the photo of the pair of them grazing with unmelted snow on their backs, they were so well...
I like lessons where the instructor explains the why, not just ‘ do x, y and z. My best instructors both have no formal qualifications but are sublime riders who have also ridden my horse.
My brain cannot process the nowadays ubiquitous ‘mental pictures’. Eg ‘imagine your head is a balloon but...