I've been doing it for nearly two years now and it's the best job I've ever done (hardest and knackeringest too, mind).
The best tip I can give him is not to get hung up on being a man doing a what has traditionally been a woman's role and politely ignore anyone who does (get hung up about it). Like any parent on their own during weekdays, building a routine involving toddler groups and planned activities aids sanity-retention. Toddler groups can be daunting if you let them when you're the only guy there, but mucking in and being seen to be someone who loves playing, interacting and just being with his child can break barriers and get you accepted. Good relationships with friends from DW's antenatal group have also helped me. There are some home-dad (that's what they're calling themselves -at least it's better than SAHD [stay-at-home-dad]) groups in some areas, but I'm not sure that isn't a bit sexist - like Mother and Baby Rooms.....
Re Mother and Baby Rooms - sorry but if the babychanging facilities are in the Ladies, I use them. Never had a problem yet.
There are guys who crave male support/friendship while homedadding. I find a couple of swift halves of an evening with a mate or two fills that gap. During the day, it's generally more helpful for everyone if I'm talking to someone who can tell me what she uses for her dd's nappyrash, or why she thinks her ds isn't sleeping at the minute. And surprisingly there are some Mums who like talking about footie and rugby!
Homedad is useful for bloke-to-bloke advice if he needs it.
It sounds really trite, but even now it's still a real privilege to be able to do the primary-care-giver thing, so he should make the most of it. And if you have any more dd/ds's, and if funds stretch, you get the delights of doubling up during the next maternity leave - which we're doing at the minute - I'll leave it up to you whether you look on that as a bonus.
Oh and the best advice he can get on childcare is here on this site, so daily web access is imperative.
If he wants to talk directly to me about it, I'd be more than happy to do so.
Good luck.
Bear