I totally get your concerns but honestly mate, do a bit research as its by far the best thing to take without delving into taking gear. I've used it on and off for years and i mean years and the gains in strength and size are very good.
Im using now actually as i had a niggly shoulder injury and it just helped take the strain a bit until i worked my way through it.
Like some one said on the thread, just stick to the correct dosage and he will be fine.
Keep encouraging him mate, gym doesn't half help your mental health too believe me
ive been hoying iron about for over 30 year.
your uncle is obvs having a midlife crisis mate