I would keep hiring. This guy may be the reason new people leave. I would ask any one that leaves directly if he has said or done anything to cause them to leave. Any way I would keep hiring. It is your business not his, but he seems to be running you. He has made himself so valuable you cant live with out him and that's not a good thing. Take control and manage your business. I'm not saying he must go, but you need the money more than him. If some one gets between me and my lunch money then we will go around and around in the mudd, the blood and the beer. You also need to watch that you are not expecting too much from new people. Doing away with the over time will pay for a new guy. Paying some one through the winter months when they are not working is not a good thing. If he cant live with the pay rate and you cant do better then it's time to cut the cord. May be he can help part time.
Work hard and make it look easy