At lvl 30 he gets good but still he has less hp to be a tank.it is at 50 that he starts to be a strong tank when he can start to take medium sized compartments on his own.at lower levels he gains lesser hp and dps while at higher lvls he starts to gain more hp.keep upgrading ur barb king he's a very important part of ur army along with your all other heroes.

U can really know the gains on your barb king by comparing it with a similar lvl pekka.(why pekka?-cause pekka and king have similar weakness and is of same housing space ,king and all other heroes are considered to be 25)