If the player's are all having fun and the only one bothered by it is you, then I'd say it's not a problem. I would privately ask the other player's their opinion on the matter. Different players enjoy different things. Maybe the ranger enjoys carrying the group and/or it fits their character well. Maybe the others are more interested in roleplay than combat and they didn't notice/don't care. Definitely find their point of view. Then ask the ranger if he feels he should be challenge more.

Depending on the answers lays what you should do. If one player feels they want to be stronger, but the other two don't care, then sure give them a magic item and no one else. If the ranger feels they want to be challenged more, then make that player's reputation "higher" than the rest and as such is a higher priority target, perhaps even sentient monsters put a bounty on his head depending on what you're fighting. Like a vampire heard this ranger is killing more undead than anyone else and wants him dead for a price, the other's don't matter. That would justify targeting him, and if the player is smart he would realize he needs to tone down his heroics.

Another option to make it difficult for him would be to look at saves. What his weakest save compared to the rest of the party? If he has one, make an encounter that challenges that save. Like if his weakest save is will, and paladin and wizard have high will saves fight a mindflayer.