Ditto, you need to nip the kicking out in the bud, regardless of the reason.
1. My first thought is now that he's feeling better and gaining weight, his real personality will start coming out.
2. What a horse eats makes a huge difference in how they behave - believe me I know - I have a horse with oat/corn/soy allergies. He used to be a Jekyll-Hyde, nasty man when he was eating grain & soy based feed.
3. Buy yourself a buggy whip. They serve many useful purposes besides tapping a horse on the butt when it's being driven
The buggy whip will be your friend in many situations
3.1 A buggy whip makes an excellent Long Arm Of The Law - an extension of your own arm.
When the lad gets ahead of you and kicks out (as he surely will), have that buggy whip ready for action before he gets ready for action.
The second that hind leg comes up and out, whap it as hard as you can. You don't have enough muscle to hurt him but you will get his attention.
You can either remain silent and let him think the Boogie Man reached right out of the ground and tried to grab that kicking leg, or you can whap him and say "NAH!" in a loud and ominious voice.
At this point he will have a startled look, maybe shake his head, and move ahead.
It's only five of the 30 seconds of "I am going to kill you" but it's very effective.
4. My food allergy horse can also get pushy at the gate - even after 16+ years of me telling him, he is not allowed to rush the gate. When opening the gate, I keep the buggy whip tucked under my arm. When he starts to push me, he gets the end of the buggy whip poked in his chest pretty good. I never say a word, I don't have to, he knows full well what he did and he immediately backs off, to politely wait until I tell him it's ok to come thru.
5. Buggy whips are good to "go ahead of you" in the tall grass, in case there's a big spider web smack dab in the way you want to walk
6. Buggy whips or riding crops are handy to have standing in the soapy bath bucket if your horse wants to kick out during bath time. It's a wonder my food allergy horse has back legs. He still will slightly raise his hoof just to test me (after 16 years:( but these days all I have to do is say "NAH!" and he puts his hoof At Ease.