Welcome to Just Answer. I'm very sorry to hear about your guy's situation. When a penis stays out - it could be due to a physical erection, but this should go down pretty quickly after the stimulation is over. If it stays out for a prolonged period of time - over 10 minutes, then this is a situation that is called a paraphimosis. Basically the penis is stuck outside of the sheath. This could be due to a narrow preputial opening or due to something (hair) being wrapped around the base preventing it from going in. This is a very, very serious situation as the penis being out for an extended period of time is very painful and can cause actual damage to the tissue. I would recommend getting him into an ER clinic or getting your vet to see him immediately.
I'm glad that he lapped it up. Hopefully he's feeling a little under the weather from the stay at the day care. I've seen numerous ones that have been playing so hard and not sleeping that well when in a situation like that, that when they do get home they crash. If his penis comes out again, it is most likely due to stimulation. The best thing to do is to remove him from the source of that stimulation - the female? - and see what happens over the next 10 minutes or so. Usually it will go in. If it doesn't, keeping it moist with cold water and getting ahold of your vet to get him in for getting it back in is recommended.