Usually they grow around a pound per week so I'd say he's on the small side right now. Usually they have most of their adult height/length by 6 months or so but will continue to fill out quite a bit. My cardi didn't really fill out until he was 2 or so.
Ein (who is a pembroke) is about 18.5 lbs at 5 months. I would say that at 6 months he is on the small side.
That sounds a lot like Ace's age/weight, and he is now 2 years old and weighs 24 lb or 11 kg.
How much is he eating? Maybe I'm thinking of someone else, but wasn't his sire Picasso? Not that a normal sized dog can't produce smaller offspring, but it's a little unusual IMO.
He eats regularly. He can be a bit pickey now that he is teething, but nothing to worry about. I'm not sure who Picasso is-you must be thinking of another fluffy blue merle!
Isn't he from Arbennig Cardigans? Picasso is a Best in Show winner from that kennel, quite a fabulous dog. I looked at the same litter when they were younger and I thought that was the sire, but perhaps it was one of their other blue merles. They don't seem to have the litter page up any longer.
Ace comes from two very normal sized parents and his sister Lulu on this site is definitely well within the norms of Cardigan bitches. But Ace is so shrimpy, he doesn't even make the minimum weight for Cardigan males. :-D He is a food lover through and through and we definitely fed him as much as we could without him being a fat tub of lard. Perhaps some just stay small?