If you're able to try and reach out and offer him a last chance before doing this I would try that first.
If you know the town he lives in you may be able to find him on the electoral register. You can do these searches online, however he might be on the edited register (which is opt-in) and therefore the best way might be to go into the local council office and ask to see the full register. This is fully legal and normal.
If you wish to take him via the small-claims route:
Step One: Prove it's not a gift. So if you're both on record in the texts stating that:
A) You are loaning him this money on the understanding that he is to pay you back, and
B) He agrees to this and states his intent to pay you back.
Then it is demonstrable that it was a loan and not a gift. Tick.
Secondly, since you have it in writing, it's much easier to prove that there was a contract made between the two of you. And therefore we ask if it meets the four main criteria of a contract - offer, acceptance, consideration, and intent to enter a legal agreement. The texts should demonstrate these criteria if they show you offering the money, him accepting the offer with the proviso of paying it back, and the fact you have this in writing going both ways.
Therefore: If you wish to take it to small-claims then you may do so. For amounts up to £500 there's a small fee of £50 if you do the paper form, or £35 for the online form.
You'll want to have copies of your bank statements showing the payments were made (originals are better - if you only have online statements then go into your bank and ask for them to verify (sign) a statement from the appropriate timeframe).
Take all relevant texts - these are exceptionally useful. It may be worth transcribing these (with numbers, and timestamps) for ease.
You may need to show your income - if only to prove that £500 is not the kind of money you hand to a mate in a birthday card.
You may be asked to prove that this is a one-off and that you've not made other large gifts to this person before or since. Bank statements will cover it.
You may also be asked to show that you've attempted other means of recovery first. If you have other records of you asking for it (and him telling you to go away) or offering a payment plan, etc, then that should suffice.
As always, your local CAB office will also be able to offer advice on this kind of thing.