Can Jewish men wear their cap in the temple?
I confess I am a bit confused by your question, none the less I will try to help.
First, I can only picture few times a non-LDS Jewish man would be in the temple:
1. Open house = cap okay
2. Waiting lobby, before entering the temple = cap okay
3. Pre-dedication construction worker = cap okay
If the Jewish man is an LDS convert, then if I were his bishop, I’d be a little concerned about the continued wearing of a yarmulke or other items associated with Jewish religious observances. That was an issue in the primitive Church that Paul and the other apostles had to deal with. It was one of the contributing factors to the Great Apostasy. Christ fulfilled the law and the observances of the law are no longer necessary. Someone who insists that they should be observed has not fully embraced the belief that Christ fulfilled the law, every “jot and tittle.”
I would have similar reservations about a converted Catholic who continued to pray the Rosary. Holding on to Catholic worship practices would indicate a lack of acceptance of the Lord’s message to Joseph in the Sacred Grove, that all the churches were false and to “go not after them.”
In either case, if the new convert was continuing to observe false practices of a former religion, I’d have trouble authorizing them to receive temple ordinances.
I hope that helps.