I've never actually tried marching with a picc, but I do play trombone, I have marched, and obviously I've had piccs. Thats the closest thing to experience I have.
I would suggest a trigger horn if one is available to play, hitting the f trigger with your left thumb is easier than stretching out your right arm for 6th or 7th. When I have a picc I have trouble with not being able to fully extend my arm, I have the arms of a primate so I can still hit 7th without reaching out my arm all the way.
If it isn't causing any pain, and the dr doesn't have a problem with it, get out there and get that marching band tan. You know, only one side of you is tan, either the back half or front half depending on which way you face in practice, then the tan lines at your socks, shorts, sleeves, and don't you just love those hang around your neck drill charts that leave a line on your neck too? ha ha, been there, done that.
The right arm doesn't actually hold any weight if you're doing it right, it's just sticking out, your left hand holds the horn.
The only other thing I can think of is be sure and stay really hydrated (our director let us keep a camelbak on while we're on the field, it stopped the whining for a water break <img src="i/expressions/face-icon-small-tongue.gif" border="0"> ), mine had yellow gatorade in it instead of water, shhh! lol I always stayed exhausted in marching 2 a days, but to beat the heat we started at 6:30 in the morning, then came back for an evening rehearsal. Nothing is hotter than artificial turf in the afternoon Texas sun, I'd rather stand on blacktop, at least that doesn't smell like a tire.
I really think he'll be fine, let him get out there and try it, if it's too much he may need to maybe march without a horn for a bit, so the band isn't learning drill with a hole