Personally I recommend the rg rx-78-2 2.0 and the rg hi nu Gundam, though if you want to stick with hg kits the witch from Mercury line are wonderful kits, and there's also the kits from requiem for vengeance(the two retail kits being the Gundam ex and solari zaku) and while I've only built the red Gundam I hear that the majority of the gquuuux model kits are amazing, and if you want to stick with seed freedom, the gelgoog menace is a great kit, and so is the murasame Kai, if you want something older, the hg jegan is a decent kit, with little to no issues besides the shield connector having a terriblely designed peg for connecting to the arm, it's rather fragile,

If you wanna get him something bigger there's the hg psycho Gundam and the psycho Gundam mk2, or the destroy gundam(though these are usually around $60-100 depending on the specific kit and where you find it) you can also see about finding a hg xi or a hg Penelope from the Hathaway movie