A sheet will stop his natural ability to stay warm more than it will keep him warm so I would avoid those. I live in TX and it doesn’t get very cold so we blanket when it’s pretty warm; in the 30s or in the 40s if it’s raining. I prefer a med weight waterproof turnout. Just be sure you are consistent. For example, if you blanket at 42 tonight, you need to blanket him every time it gets that cold. Also be sure you can unblanket before it gets too warm, some horses can colic from the cold to hot temp change.