First issue is bandwidth.
He is going to be using up your bandwidth. If connected at the same time he will be slowing down your connection. If he only does general internet browsing and checks emails etc that probaby isn't such an issue. If he starts downloading/streaming a ton of media, it becomes a problem. Especially if you have a bandwidth limit on your broadband package. You could end up exceeding that limit and being charged for it.
Second, and tbh the biggest issue, is security.
Giving him the WiFi code is giving him access to your local network. Depending how you have things configured and how computer literate he is - he might be able to gain access to your files.