How to gain muscle the natural way?  

We don't gain muscles because we are already blessed with natural muscles from our Creator. If your husband is into body building championship competition, then products which helps in the growth of muscle hormones like proteins would be helpful. However, if your husband is only concerned with a toned muscle, then the natural exercise like lifting weights is just enough. Natural is always best. Good day.

Lift weights?

before i'll give you my advise, may i know what's your husband goal to his physique? toned muscle? or serious guy to step the world of bodybuilding career and compet later?...generally speaking, your husband needs to convince his muscle to grow by using routine that his body isn't use to it and use calorie cycling to his diet, have lots of rest/recovery (9hours sleep), supplements are essential depending in your husband lifestyle, if his job is not demanding and he can maintain his 6 meals a day (every 3 hours) it's ok not to have it, consequently, if his job demanding, he needs supplements (whey/casein protien, MRP, NO, glutamine)...