The recommended weight needed for hibernation is 450g, but that is for a fair bit later in the year. It sounds as if the little one there will have plenty of time to put on at least the extra 20g needed. It depends on the weather a bit, but it probably isn’t until about November that the weight becomes more important. I would leave him alone and as long as he looks as if he is growing, I would let him live a wild undisturbed hoglethood.
Glad to hear you keep your dog on a lead for night-time excursions – very wise.