Giving him higher level items.
You give him a level 11 item, it's super powerful, but you know that in 3 levels it'll be back to normal. So you can then give him another higher level item, and it's never ending.
That's the best system I've found when I want to make items sexier.
Items above level are good.
Items that let him do things he otherwise can't. Those are somewhat hard to find in PF2 if you're not giving him above level items, because static DCs and limited uses per day make them lose their luster quickly.
But if you give above level items the DC will make the item potent because it's high level.
And you could also make somewhat custom items. Look for effects that already exist in game, and add them to other stuff.
Like what if his armor granted him flight. I can't even recall if any armors in the game grant access to flight, but if they do you could give it to him early with limited uses. Perhaps allowing him to activate it as part of an action to move (action economy adjustments are probably one of the best ways to customize something).
You could also look at completely custom items that focus on the character's abilities. I jumped to chronoskimmer because it seems easier to come up with items that would feel powerful.
Looking at the feats open to chronoskimmers, there is the borrow time ability. Perhaps you could give him an item that allows him to "store extra time". So instead of only getting quickened 1, he could get quickened 2, but then would be stunned 2 on his next turn.
How does he feel about consumables? Some people are cool with them. Some people (like myself) have difficulty using them at all. If he's in the former group, then giving him higher level consumable items would let him have little bursts of obvious greater power without unbalancing the game overall.
Of course, you also have to consider the balance with other players. If you're giving this guy special bennies with extra juice, and you're not giving the same to everyone else...
There's something else that I might suggest would *be* a homebrew, but one that could still fit inside the power budget. Make items that you can invest feats into. Like, it's some sort of soulbond thing, and you can pour some of yourself into it to get greater effects. Then you can have the overall effects paid for out of the budget of both the item and the feats. If you lose the item, you have to do something to untangle yourself or whatever, and then you can retrain the feats back out.
Finally, if you have the ready cash, you might want to take a look at Battlezoo Bestiary. It's got a monster parts system for crafting your gear as you go. I haven't experienced it myself, but I have every reason to expect that it would be reasonably well balanced against the PF2 baseline, and it's a very different approach to your base gear that could render up some cooler effects.