I could very well be wrong but:
I don't think this is a neutering issue, but just the stages of being a puppy. When my boy was about that age he played very rough with our doberman. Granted, our doberman was more his size rather than a jack Russell. Our dogs would get rough to the point of knocking stuff off of tables or even knocking over tables completely. No matter what we couldn't get them to stop playing. They were best friends and would have been forever. I think its just the bond he feels for your dog (your jack is teaching him things in a dog-dog way) and the play of a puppy who has no idea what size he is.