Do they have the money to pay please?
Oh that won't be a problem. This is a large sum of money and, although it is your son, a court is not likely to deduce that there wasn't an agreement to pay. Anyway there has been a repayment and also you can rely on your emails as evidence.
Getting judgment won't be an issue.
It is getting the money that is harder.
The place to start is to send them a letter before action though. Give them a 28 day period to either pay or make arrangements to pay.
Make clear that you will sue and seek costs and interest if they do not.
If there is no result then issue against them here
If they still don't pay then you can always seek an attachment to earnings. That is fairly cost effective and works quite well for an employed person.
Can I clarify anything for you?
All the best.