Your cute baby has East/West Pasterns (referred to often as an eastie/westie). This condition is usually due to inadequate diet. The majority of east-west pasterns have less to do with genetics and more with nutrition. Sometimes, this condition is temporary and the pup will outgrow it if his nutritional needs are met with a correction in diet. However the condition, if prolonged, carries a risk of putting excessive strain on the joints and inner legs, hastening the structural breakdown of the bone plates. This could ultimately lead to arthritis in the joints or other degenerative joint disorders.
You should change him over to a very high quality diet and also have a vet take a look at him in case the issue is in fact genetic. Also, make sure that if you have slippery floors (hardwood, linoleum, etc) that you lay down some rugs so that he can get good footing. Good luck! He sure is a doll!