No it isnt nearly enough.
£100 a day is dead easy to earn on your own round with no effort at all. Before long he will realise this, and appreciate the fact that he could be doing houses like these himself and keeping the whole whack.
I'd be happy to earn about £70 from a worker, so I would offer him £180 per day provided he reaches the minimum target each week, which should be resonable and allow for factors beyond his control.
You must also give an incentive to stay working for you, rather than setting up on his own, such as paid holidays, paid sick leave, pension scheme etc. Take him on as an actual employee and pay him via PAYE, that way it will be like a proper job, not a loose arrangement. Nothing worse than when people take someone on on a self employed basis - that is a recipe for disaster.
£20 an hour is NOT good money in window cleaning terms, and anyone who thinks it is needs to get their act together.