I have an issue. I have the archmage robes, but they serve no real purpose for me and I want them to go on my mage companion, Marcurio. I give them to him but he doesn't equip them, because he's wearing the generic robes he automatically wears that isn't even in his inventory, but he doesn't get the huge bonuses from the Archmage Robes.

Is there some way for me to tell him/get him to wear the better robes?

no companion wearing cloth will equip other cloth unless it has an armour increase, the way they are programed is to wear whatever gives the better armour rating or damage (base value, even if you upgrade it they treat it as base), that being said I gave him some thalmor robes and every now and then after going through a load screen he would decide to put them on, and then not take them off as long as I didn't screw with his inventory or anything like that.

also, his base robes pretty much give him the same benefit, the only difference is for him is aesthetic.

Clothing on followers is kinda iffy; maybe if you give the follower clothing that specifically boosts light or heavy armor (whatever they'd prefer) they might wear it?
I've never tried this approach, it's just an idea.

I think what the NPCs wear is determined by which items are included in their Outfit. It doesn't matter what you put in their inventory, they will still wear the outfit. (doesn't apply for weapons and shields)

I usually found that mage followers will change robes upon zoning between cells, or sometimes fast traveling. Although it is fairly iffy and I haven't used a mage companion in a while :/

Yeah I have the same problem with Mjoll, I give her a daedric bow yet she still uses a hunting bow any help on this.

If you are on pc you could use the console.
showinventory
will tell you what items (even hidden) that he has and their item id and how many, then
removeitem itemid #
will remove # count of that item. I used to use that with Aela to get rid of her hunting bows so she would equip the glass bow I made her.

too bad im on xbox

Give him any piece of light or heavy armor and he'll equip it. Then pickpocket his default robe off of him. Trade with him, take your light armor back, then give him the archmages robes.