155g isn't necessarily too small, there are hams who are healthy at that size especially in america where hams tend to be smaller anyway. But if you're concerned he's a bit thin then there's no harm fattening him up a little bit
it's hard to get a ham really overweight since they're so active.
Just remember to ease him onto a new food over maybe a week or 10 days, feed both and slowly increasing the proportion of the new food. You do have to be careful with rodent foods, apparently any brand can just write "complete" on the bag whether it's good quality or not