Well if you want to be effective and have neat and clean clip, first bath him! Really scrub with soap and warm water! Keep a blanket over him and clip with the next two days.
Second figure out where he sweats the most. So most sweat a lot on the neck, chest and saddle area.
You can look up different clip jobs, and then tweak it to how wide to keep your clip. You can leave his hi d quarters fuzzy but clip the flank area and belly go as high or low on the ribs to shoulder, you can clip the whole neck or leave some hair near the mane if your worried of accidentally taking some of the mane off.
Really got to evaluate how hard, many times a week you will be riding and how many blankets you want on him.
Good luck! If you where near me id help you out and show you tricks!