I *think* options 4 and 5 don't work all that well. Under 21 teams usually play in leagues with a very low reputation, so game time won't be seen as being at a "competitive" level. I might be very wrong here, but I think I remember reading somewhere that game time in the U-18's is worth more for their development than the U-21's?
Either way, if you put him in the U-21's and make him available for the U-18's, he's going to miss out on valuable training time due to schedules not alligning. I'd probably look for a way to force him into a playing position in the second squad!