was he stabled, or out 24/7 while you were away?
after 3 months i would not be cantering the fdirst week, even if he has been out, and absolutely not if he was in overnight. they do keep themselves fitter out 24/7, and regaining muscle is faster than building it up from scratch, but even so, asking too much can put him at increased risk of pulling a muscle or straining a tendon or a ligament. if he trips or stumbles, he is more likely to injure himself more seriously.
they don't know their limitations, especially when factoring in the weight of tack and rider, so we can't rely on how enthusiastic they feel to gauge fitness.
i woudl take at least two or three weeks, riding every day, to build up to canter work.