How can I help one of my players be more tanky?

I’m running a game of four (currently) 5th-level characters. All of them are heavy damage-dealers (a wizard, warlock, ranger and paladin) with the paladin as the nominal tank. However, the paladin is consistently going down the first few rounds of every combat. He has a pretty high AC (20 with shield of faith), but low constitution, and the ranger (a dwarf) and even the warlock have more hit points.

I can sense that the paladin’s player wants to be more tanky, but isn’t really sure how. They are already in the habit of using shield of faith, as I mentioned, and took the protection combat style. What kind of magic item/special ability/other help would be good to give them to help them feel like they’re achieving their goals, without just having them stumble upon the Amulet of Health and letting them dominate?