I'd never want to stop a company growing, even my competition (as they grow stronger, so does the market - therefore you - in most cases) though I'd perhaps consider waiting until you're taking on work yourself (I read you work for a building company yourself?).
It would be a shame to miss out on a good employee, he's clearly keen to learn, so I wouldn't expect much other than that - he'd be asking a lot and could be slowing you down - but you're investment in him will pay off over time. I just think you need to be getting work in yourself to ensure the workload can be split when he starts to get better which means there is a chance you can earn from him. Though if he feels he can get the work himself at that stage then you need to be sure you've built up a good relationship with him and you pay him fairly.
It's easy as an employee to think you're being taken for a ride but if they did a full week (of a
tiler getting his own work in) they'd see where the extra cash could be spent. Quoting / advertising (or your own kind of PR) are both things that can take up a lot of time for the avertage
tiler with no return. So just make sure if you do go down that road (assuming you want to get out of your site-work for the large firm and start running your own business) you protect your investment and either be straight up with him and show him what it's costing and what you're both getting out of it or manage him well and give incentives and keep him as an employee.