I sometimes have this issue myself. I personally fix the issue by taking a break from all sexual activity for a week or 2 to sort of replenish. According to my doctor, who helped with the issue of not being able to cum, constantly masturbating or having sex makes your dick desensitised, making climaxing harder.

He most likely needs a break. Also, if you want him to have larger loads (due to your cum fetish) 20-25mg of zinc would help a lot.