First thing is a standard health exam. If he is healthy you can get his testosterone levels checked.
The key natural things to do to help raise testosterone is all the good oils for the hypothalmus. Omegas and evening primrose. Zinc 50mg or so a day. Too much can be real bad so keep it at 50mg. Raw glandulars for the hypothalmus and pituitary and adrenal.
Avoid adding testosterone, it is better to try to get the body to make it.
There are tons of sex herbs out there some work others don't and some have side effects.