Deepsea

I am a British citizen but been living in Hong Kong most of my life. My son who has recently finished university would like to borrow money from me to purchase his first property in the UK. This would be drafted up as a loan between my wife and my son and would need to be paid back after a set period of time. Please can you let me know if this contract needs to be drawn up through a solicitor or just between ourselves, and could you also let me know if he would need to pay tax on the money we loan him and if so, how this is calculated.