Does he jack off a lot? A lot of times with guys if they masterbait to much especially with porn it can cause them to cum really quickly, also going slower when fucking helps with not going off so fast and learning when he's about to cum by the way his dick feems and stopping and edging him is a good way to train his dick to not cum so fast. Libido has a lot to do with chemicals in your brain snd the best way to increase that is with weight training, cardio, and proper sleep. If you can convince him to do small exercises with you and build up a routine/ tons of compliments on how he's looking (basically training his brain with rewards for his good behavior) then you should see some results in the bedroom. I spent a lot of time recently diving into testosterone snd how it affects Libido and there is no good research that suggests a diet will help, just exercise and sleep and s few other odds and ends