My friend and I have taken her young son and a few of his friends out mountain biking a few times. They're 13-ish, so a bit younger than your friend's son. What we discovered is that even though they're boys, they're not in cycling shape. Furthermore, they needed to be cued to shift! They would try to climb hills without changing gears, and that simply didn't work. Your young friend may need some advice on gearing and how to pace himself on hills. It sounds like he's a total newbie, so you have to assume he knows nothing and teach him. Be patient and tolerant on rides with him, and keep them short (maybe 10 miles max) and build him up gradually. Then drop him off at home and you and your friend can pick up the pace and do your training. You may have to accept that rides with the young man are not going to be work-outs for awhile, but they can still be pleasant experiences--bonding, shared memories, etc.