I'm baffled by what you mean, if I'm honest, and I've upgraded a lot of phones!
The handset is cheaper on upgrade deals, if I've understood correctly.
For contract phones, the handset is usually a lot cheaper on upgrade, because your contract is about to end and therefore you can leave. Companies offer the phone cheaper so that you'll stay, and agree to a new contract. It's guaranteed income for them. People wouldn't be able to give you their upgrade, though - it'd be linked to the account. Some people do accept an upgrade offer and then sell the phone on eBay, but it'll then sell for the usual amount - they make the profit on the phone price. If they upgraded it and sold it to you for the upgrade price, they'd have agreed a new contract but got no benefit.
For pay-as-you-go, upgrade deals are often so rubbish that you might as well as just buy a new phone and swap the SIM over. You don't need a code to upgrade, though - you need a valid account that is reaching the end of it's period.
So what is that you're looking for, and what phone does DS want to get? You can usually try and hunt out a deal on eBay, but you do have to be careful because if it arrives broken, it's a hassle, and you don't have that much time to be posting stuff back and forth.
I agree with everyone else about letting him spend his money, too. I'd give him £20/£50 and tell him that's his budget, and he needs to buy something within that. It'd be very mean to let him spend all of his money on a present for your DH, however much he wants too...