ITA with Gandalf in saying to neuter this cat ASAP. His hormones are
developing and spraying is a natural part of that.

Do you only have one litter box? Some cats prefer two, one for urine
and another for stool (ours do).

Also, make sure to play with him often. Kittens have a lot of energy
to expend, and can get into trouble if they aren't mentally
stimulated.

Rene