I don't employ, so am speaking here only from a view having been in commercial business other than window cleaning for 30 years.
As I see it the considerations are:
1. Do you NEED to make more money than you are doing now? If you want to make more money than need to, then keep him.
2. If the weather changed next week to that which is more usual would you prefer to have him and indeed could you, at that point, make more money than you could if there were the two of you? If so, keep him.
3. If you let him go where would you stand with regards to finding a replacement of a similar standard; reading on CIU it seems you have a diamond.
4. Are you prepared for the guy to set up on his own if you sack him; if he's got the nouse and a good work ethic that what he may do.
5. Are you prepared for the possible legal implications; check if a spell of bad weather are grounds for redundancy - it probably is if your work overall has dropped, but if he starts to fight it.... If you've not been scrupilous in your paperwork and all dealings with him he might have some sort of other case against you.
6. Are you prepared for the actual difficulty of saying, "You've are great bloke, a good worker, but you're sacked cos it's raining".
The weather won't last. Why not get him out cleaning whilst you canvass, or get him doing wet weather jobs like gutters and soffits, pressure washing. Communicate with him - tell him the situation and you'll have a good worker for life.
Whatever you decide, good luck.