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It does sound as though it may be age related & they do sometimes tend to lose weight as they get older, you're doing the right thing with what you've been feeding him, I would try him with porridge too as that's really good for him, small pieces of nut & pumpkin seeds are good too.
He may need to continue with extras to maintain his weight, he won't need so much protein now so try not to give too much in the way of high protein extras & if he eats quite a lot of soft foods or seems to be going off the harder foods keep an eye on his teeth just in case they start to get too long.