I will be honest with you: the only thing that is going to stop the sweet itch is a combination of a good, high coverage rug which is light enough and breathable enough that they won't sweat, weekly (at least) baths using mixture of dettol and an insecticidal shampoo, a homemade flyspray using essential oils as well as vinegar, tea and meths, killitch after the bath and one other time a week all over the mane, tail, around the belly and legs and down the spine (NOT on the sheath, sudocrem/vaseline/itch-stop cream on the sheath and muzzle and any other exposed areas. Also, if you can, keep him in as open a space and as far away from natural water sources as you possibly can (high on a hillside is best

).
Sorry to say that, but it's my experience. Last year, my boy was owned by our friends still. He's a big lad and they struggled to find rugs to cover his belly. He was in a rambo sweet itch hoodie, and his belly was red raw, his ears were read raw (as couldn't keep a mask on him that had ears in it and the hoody has none), his sheath was red raw. He was kept in a sheltered paddock next to a stream with lots of tree covering. He was sprayed before rides using a normal shop spray, and they used itch stop all over his scabs. He was in during the day and out at night from March to Sept (when we bought him) due to the midges.
Now we have him. He has a snuggyhoods rug and hood set which covers everywhere except sheath and legs (I bought the legs but they are flippin' USELESS!). I put sudocrem around his sheath area. I had a muzzle guard built into the hood so that it protected his muzzle as well, but I put sudocrem there as it doesn't cover the whole area. I also had a fly fringe built in. There is a built in tail guard and built in ears. He is out 24/7 at the moment (mainly to help my cob, who's resp problems have recently got worse), which has helped him no end as he doesn't get as bored and there's not so much readily available for him to scratch on. He is sprayed once or twice daily using my home fly spray. He has a bath once a week and is plastered in killitch after that bath and one other time a week. He is groomed regularly as loose hair will make him itch more as well. He is in a very open, large field with no natural water source nearby.
In April, I had to go 2 weeks without a rug as the seem was faulty and it had to be fixed (free) by snuggyhoods. During that 2 week period he did develop two very small patches of sweet itch on his belly. Now the rug is back on, these have cleared up. The moment he gets ANY scabs, he is covered in sudocrem in that area to soothe it and to stop any further biting.
Apart from those two little patches he is free of it this year, for the first time since he developed it, aged 3, 4 years ago. The first two years they had him (before they started taking it seriously), he was covered head to toe in scabs and lost lots of hair where he itched. Last year he was better with the rambo but it DID make him sweat up a lot and just looking at him in it made me feel sweaty (the inside is a plasticy material). Last year he also lost about 75% of his tail and it's taken til now for it to grow back). This year he has a full tail and so far, no itching there.
Some people go for the injections. Lanky had them in his last home and they did not work for him but, until this year, he was a particularly severe case. I know it is eatly yet (still 4 months of torture left and worse to come yet) but I am quite proud that the changes I have made, have meant he is much more comfortable and almost sweet itch free!
So in short, get a rug which won't make him sweat if you haven't already, as once you remove the rug and he starts getting bitten (particularly in this damp but muggy weather), you will struggle to get back on top of it.