First of all, there maybe be some existing Wifi access that your son can access with his PC. Set the thing to look for wifi and it will come up with all it can find within range. It could be that your ex has a nearby neighbour
whose wifi he can access. If this neighbour agrees, than this could be the answer.He will need the access code though.
It could be that this nearby wifi hasn't got a lock on it, so making it even easier. Don't forget, it doesn't affect the existing user if someone else "taps" into their router. He may also find a BTfon site or similar. He can also get wifi in places like McDonalds, Burgerking, or a branch of Weatherspoons (sorry...not sure how old your son is...maybe not old enough for pubs ! ) All these places do not make a charge. You may have noticed how popular McDonalds have become lately, with all those smartly dressed businessmen using thier laptops and nursing a cup of tea for what seems like ages !
The alternative is to use a wifi dongle. I use one from the Three Network when I am in my caravan in Cornwall and it works fine. If your son just wants to browse the 'net and see to his emails, than they are quite reasonable in cost. But as soon as he starts downloading stuff, like youtube for instance, its gets a bit pricey. I bought mine is a branch of Currys and it wasn't very expensive. It comes with a sim card, so works rather like a Mobile phone. You can buy upgrades on the 'net, and these top -up immediately.
All this maybe sounds a bit basic but I hope its of some help !