Also, give him a good dose of Vitamin A if your grass is not up yet. This helps keep away ichies, rain rot lice and a myriad of other skin issues that always crop up this time of the year. I learned this from a very accomplished horsewman and since starting it, never have skin problems or lice ever anymore.
I give my full sized horses 5 CC of the injectable cattle vit A and D (found at TSC or any cattle supply place for about 15 bucks which will last a long time for one or two horses) Use it ORALLY in the feed once a month starting about November till grass or fresh hay is in. Just squirt it in there.
If they have any problems and im not just doing it preventatively (Only new horses coming in ever have problems anymore since I started this) I follow up with 2 CC each week till its healed or gone. You can also put them on a supplement that has high Vit A.
This time of the year there is no Vit A left in the hay and the horse has no reserves. This is why so much of these skin issues crop up till green grass comes in.
Ever notice that once the green grass comes in, all the lice, rainrot and skin issues pretty much go away whether its treated or not? The only time horses have these problems during grass season is if the horses has been in a stall or dry lotted on old hay. Fresh hay has vit A for awhile.