It's a common thing for men to have night erections. It could be due to physiological reasons or any dreams/fantasies that they have while sleeping. It's normal to fantasize about own wife/other women as it makes a person feel desire to have sex. It's not possible to control the content of fantasies/dreams while sleeping. If he's not actively drawn/involved with other women then you need not to worry about his source of night erections. You need to worry if you find him attracted to other women, not involved in the marriage etc.
Next Steps
See a counselling psychologist if you have any other reasons for doubting the fidelity of your husband or you tend to feel insecure often.
Health Tips
In fact if you look at it from a different perspective, he's being honest with you and not hiding the real reason.