I think he might be worrying too much, as you say he has only been riding a few months, Cycling produces changes to the body systems, most for the better, some for the worst, but generally for the better.
If he come back and is relaxed and only starts fizzing a few hours later, it is that you should be looking at, Sounds like he could do to do some training down, he no doubt is pushing himself physically to improve his fitness, I would suggest that for the last 3 miles on his return from his ride, he rides at a slower rate, say 10Khm, to wind the body down, clear out the lactic acid from his muscle,s, and maybe munch on a mars bar to balance up his blood sugar levels. In time as his fitness improves he will or should not have these after ride effects. However if it continues I would suggest a visit to the Doctor,s