Since you're building this NPC to support your party of three, build it as a Companion instead.
The main reason monsters are different in D&D 4e is to provide them with just the powers that would be useful in a single encounter, without worrying too much about daily resources or situational powers.
Just to make a comparison with monsters built as PCs, which was the shtick of 3.5e, an NPC could have its own special attacks, items just like a PC, class features, spells of any (spell)level up to whatever was appropriate for their (character)level. This produced weighty statblocks with many abilities that the NPC would have never used during the limited time he was on the scene.
You're not building for a one-shot monster. You're building for an NPC that will be kept in the spotlight that the game points on the PCs.
So, the first instinct would be to have it built as a PC, same level as your party, feats included.
Of course this introduces some new problems. First of all, he shouldn't steal the spotlight. Since he's the striker, it looks like your party lacks one. Focus on how the other players enable him to be their offense: it's basically an escort mission.
Second, PCs are complex machines with lots of fiddly bits. Unless your players want to communally decide what this character do, follow my initial suggestion and use the Striker Companion from DMG2.
This of course tells you that your problem is not how to assign feats to an NPC but how to build that NPC and I showed you a way to do so that's both better and avoids the feat problem.
Anyway, you might be willing to create NPCs as monsters and fall into the problem you asked about anyways. While I'd just consider giving him the benefits of the feat even if he does not get feats, my experience tells me you shouldn't - build a monster or reskin an existing one instead, because the monsters with class levels proved unsatisfactory to me and to whoever I talked with.