There are numbers of reason for loosing sexual interest. Married life of only 5 years and happy relations rules out boredom or burnt out. He may have some sexual problem like erectile dysfunction (not getting enough tightness to penetrate) or premature ejaculation (when he finishes too early). Male will always feel ashamed of such inadequacies and hesitate to accept it so he avoids. Having a son does not rule out these problems. Discuss with him about his problem of lack of interest in sex and your needs as well. Take help of psychiatrist as they are qualified sexologist too.