Provided he has acknowledged in writing within the last five years that the debt is due you can take civil proceedings against him. My concern is that the loan was made over five years ago and that could mean it has become time barred. Assuming that is not the case you can sue him in the sheriff court. That doesn't mean you will recover the debt if he has no assets of course. You will get a bit of paper saying he owes you the money. You would then have to ascertain how you could enforce the decree and get paid. I hope that helps. Please leave a positive rating so that I am credited for my time.
A text admitting the debt would do. You would enforce a decree by arrestment of earnings or bank account, inhibition against heritable property or attachment of property. Your ultimate remedy would be to make him bankrupt. JustAnswer experts aren't allowed to recommend specific lawyers but you can contact the Law Society who can: www.lawscot.org.uk
You would have to know what his personal assets are.