Taking your question at face value...
After neutering, testosterone levels wane gradually over a period of weeks - the effect is not instantaneous. The rescue I volunteer with recommends waiting at least 4-8 weeks post castration before allowing a neutered dog to come into contact with a female in estrus.
Most neutered dogs are no longer interested. However some, especially if castrated post puberty, can become excited in the presence of a female in estrus. No, conception is not possible but nonetheless you should keep your dog under control. If you encounter a female in estrus she will likely be on-lead (as is the law in most cantons), a clear sign that contact is not welcome. Owners of a female in estrus do not want random dogs pestering them - so steer clear, leave the laydeez alone.
Assuming other dogs are castrated/not in estrus, play is perfectly fine - but in general one should not allow one's dog to mount another, especially a strange dog. It's bad etiquette and potentially dangerous. Remember that mounting behavior is not only sexual - it is also a challenge/social hierarchy display, or simply generalized excitement. It can be harmless in a group of dogs who are socialized to one another - but with strange dogs, or if a previously friendly decides that your dog has now crossed a line, mounting can also trigger a fight.
If your dog starts to mount, recall him. It's good manners, and responsible ownership.
(For an example why: my arthritic elderly collie's spine was severely damaged by a young dog who ran up to him and tried to mount. Of course the dog's owner response: "He just wants to play!"

)
So to your question - assuminig his stitches are healed, a good game of frisbee is a better way to burn off any excess energy as his hormones settle.