Actually the lettings agent i deal with the landlord pays the admin fees out of the first months rent payment.
and most of the lettings agents around here it's a months deposit + first months rent, he'll get his deposit back from us, what does he have to save up bar maybe up to £100-200 depending on the cost of his new place? (if he has to pay his deposit prior to receiving the one back from us simply whack it on an overdraft until the other deposit comes back less than a month later to pay that off, it's not rocket science)
Admin fee's aren't always charged, but I've been looking on and off around the Bristol/Bath area and the general setup is:
£100-250 'referencing' rip-off,
Some (and this is the minority) put a further £50-150 on top of that as 'admin' fees
Then at least 1 months rent as deposit, more often it's 1.5-2 months.
And obviously the first months rent,
But he will lose the referencing fee's (and I haven't seen a single letting agency that doesn't rip people off with these), and potentially a further admin fee so could be anything from £100-400 down the drain,
And yes, he'll get the deposit back from your place but not till a week or two after he moves out (think they've got 28 days maximum), and he's likely going to end up having a week or so overlap to move/clean which will cost more as well.
So you are costing him money, now in principal this is fine, he's likely to have to move sooner or later and that's one of the risks of renting, but to specifically avoid telling him so that he ends up with a short period to find a reasonable sum of money just to serve your own needs is quite selfish. Better in my opinion, once you've decided you're definitely going to sell, to let him now informally but that no notice is served until you get a buyer, this way he gets some notice to start ensuring he has the money to move available to him.
This works both ways, a year or so ago my housemate and I were considering moving, we informed the landlady (non-managed, so direct contact) that we were thinking about it and she then knew that potentially she'd need to get things sorted with a letting agent again, in the end we didn't move but I'd much rather this than leaving her in the dark even though I'd be perfectly entitled to do so.