I'm hoping other moms of boys out there have encountered this and will have some good ideas: our DS2 (24 mos.) has discovered his penis, in a big way. I'm ok with that, except that he will smoosh down the top of his diaper to get his hand in, and after he's played awhile and checked that all the parts are still there, he'll remove his hand and his penis will be vertical, often with the tip aligned to the now-lowered waistband of the diaper. Thus when he urinates, it comes shooting right out the top, and all over his shirt (the diaper almost always stays dry, though I change it anyway.) This happens only occasionally during the day, but has become a problem at naptime and at night - we cosleep, and every single night I am up changing him, me, and anyone else who got soaked, and sometimes multiple times, sometimes having to remake the whole bed, depending on how many episodes he has. I've resorted to putting soaker pads under him to save the mattress, but would love to find a more proactive solution!
I do know that if I could get him into a blanket sleeper, he would no longer have access - but since we cosleep, the blanket sleeper is too hot for him and he fusses until I take it off. I've started cinching his diaper so tight that he complains - but it doesn't seem to affect his ability to get his hand in there.
Yes to the crotch extenders - love those for making DD's clothes last longer! Is he in disposables? We CD, but when DD was younger and we would put her in disposables to travel I always used a clother diaper cover on top to keep everything in and prevent blowouts, so maybe that as an option.
You know what they say... things are always going to be more appealing the more "difficult" it is to get it. We humans do love a challenge. Perhaps you should consider beginning to potty train him? Let him run around bare butt for a few days...and maybe in that time along with getting the idea of potty train, he'll eventually get bored of playing with himself like that?