He looks good and they do bulk out as they get older but some are just naturally leaner than others. Caoimhe was like this, I used to call her Teflon Ribs because nothing stuck to them
She's nearly 5 now and has had puppies but she is still very lean; she has next to no body fat at all but she is quite heavy because she's solid muscle - she's only 14" to the shoulder but weighs 14kg. Rodaidh is 16.5" and also very muscular but weighs 13kg. She needs a lot more food than most dogs of her size and always has done though, when she was trialling she was obviously on a lot more and now in the working season between the two of them they're clearing away 240g of kibble in the morning and either 7 chicken wings (Caoimhe x4, Ro x3), 2 blocks of tripe and 2 eggs, 2.5 packs of NatureDiet, 5 tins of sardines and 2 eggs or a portioned pheasant or rabbit carcass in the evening, plus a lot of treats during the day. Tilly is bigger and heavier than both of them but on much much less food.
