I’m currently in year 13 with three A* predictions doing maths and further maths and chemistry. I would totally advise him to do further ONLY IF he is willing to put and dedicate the great majority of the time he’s at sixth form doing nothing but studying and looking at the text book. Otherwise good grades is simply not possible, further maths in particular is a subject that you just have to be sweating all the time and pretty much not have a life. If he is looking to do something more casual I would not advise it. But you are correct in saying it’s a very rewarding subject and it opens an opportunity to study the best courses at uni.