Assure him that you love him unconditionally, stutter or not, that he is way more important than the fact that he stutters. Remind him that there's famous people who stutter but they didn't let that stop them from achieving their goals, and also regular people can get their sentences muddled up or add some repetition here or there but they just move on from it and don't beat themselves up for it, as it's quite unlikely a person will remember coming across a stutterer for a long time. If his confidence is really diminishing by the stutter then as others have commented, perhaps suggest speech therapy to him or better yet, perhaps he could join a support group of those who stutter as well but learnt to normalise and adapt it, rather than desperately trying to 100% rid of it when there's no cure but it can be somewhat controlled with regular practice and speech techniques.