When Rufus got spayed I only trimmed his private parts. The incision is very small (between his penis and his testicular). I don't think it's necessary to cut his hair. If you want to grow it just grow it
What you will need to keep in mind is if you want him to weer a cone or a onesie. If you choose for the onesie keep it mind that he will have to weer it for atleast 2 weeks (until his check-up) and his hair can get all tangly