I've suggested maybe as a Christmas present he could get a better one but I'm not just going to be getting him one for no good reason other than him wanting one.
Then why say he might get one at Christmas?
I think you're being a bit flaky here, if I'm honest. He's senced your weakness and fully intends to needle you until you give him and give him what he wants. Exactly as your last sentence says in your OP.
Personally speaking it would be a HARD NO from me. I would make it very, very clear that DS will not be getting a phone with a SIM until X age, and over estimate that. Then be totally non negotiable and immovable until he realises that there will be no grounding you down. And then he gives in, rather than you.
I have a 11yo, he will be 12 in September. He has no SIM carded phone. He has an old Samsung to use on Wi-Fi at home, like yours.
He is 3rd born. Like his older siblings (also September birthdays) he has always been told that he will get a phone with a SIM for the first birthday he has in Secondary school (ie his 12th birthday, but it will be only a few weeks after starting Y7).
He is in fact getting a phone with a SIM and contract next week. He doesn't know this though, it's an end of Y6 surprise. Fact is though, he has long since not pestered me for a SIM phone, even though friends have then, because I've always been very clear on the fact that he'll get one the Sept he starts Y7. That's always been consistent, never changed and any suggestion at negotiating shut down immediately.
By offering your son an option to negotiate, you created this problem.