> I’d be feeding him 4 meals of Royal Canin per day.
Well from personal experience, I'd not be feeding him ANY Royal Canin!! With my boy, bought at 4 months, he'd been on RC Junior and for me, that's far too high in protein for his breed. It may be fine for small breeds, but for big puppies - no. It encourages too much growth, too fast resulting in bone problems, as was the case with my boy. Even as I got him off and onto something with a more sensible protein level as soon as I saw what the percentage was, with RC Junior.
If he's underweight, and he could just be the runt of the litter?, then yes, give a late evening fourth meal too. And we always gave our puppies a mid morning, mid afternoon and late evening (they didn't need a full late evening meal although we did leave dry food in the box overnight) drink of warmed goats milk.