There can be many reasons for reduced sexual desire including low levels of testosterone, high blood pressure, depression and high levels of stress. You do mention that he has no physical problems and it’s all mental. If that’s the case then maybe negative thoughts and feelings are interfering with his desire. Some negative thoughts may reflect self-doubts involving inadequacy or a fear of failure on his part – this pattern of thinking could dampen his sexual drive and result in avoidance. Try boosting his self-confidence by noticing small good things he does, and expressing your appreciation for them.
Other negative thoughts may result from interpersonal problems between the two of you, leading to underlying feelings of anxiety, sadness, or anger. The loss of desire itself can lead to misunderstandings that can complicate the relationship. If that’s the case, the way to resolve it is through building a close emotional connection again. Maybe you can shift your efforts from focusing on physical intimacy, to focusing on emotional intimacy. Perhaps you can start by telling him you miss the emotional closeness you had before, and want to revive it by spending quality time together each week, where you re-connect with each other in a way that is mutually pleasing. This may appeal to him since you are in essence saying you want to be with him because you love him. By doing so you can increase the positivity factor in your marriage.
When he is thinking good thoughts about you, negative thoughts won’t occupy as much space in his head. Doing small caring acts for each other every day will also increase positivity. Remember to express appreciation for even the smallest efforts on his part. Hopefully this interaction will be reciprocal. Emotional closeness increases positive feelings, but cultivating this takes conscious effort and patience. If the pressure is taken off him, and you are engaging in good shared times together, you may find that his sexual desire will be revived.