Personally I would want to be very sure of the type of rider my cousin is. Your boy is young and impressionable, you sound like a careflull and considerate rider, are you 100% sure your cousin is the same? I'm don't know your cousin so please no harm intended, but like you said yourself, you don't want to get back a totally different horse at the end of three months. If say for instance your cousin doesn't mind if the horse moves a little while he's mounting, you might find yourself with bad habit(s) to undo. That would be the exact opposite of what you want. Maybe if you can take your boy out to the farm for say a weekend and let your cousin ride him in front of you so you can see how he rides and if they get along. Or like the OP suggested maybe a month trail period? See how it goes. One hears so many horror stories of people who lent their horses to somebody - be they friend or family- and got t totally different dog back.