Many thanks for your patience and sorry I could not reply until now, I had some travel issues after court.
Anyway going back to your query, if you wanted to reduce his salary then in the absence of a specific contractual clause allowing you to do so, you wold be acting in breach of contract. So you cannot just go and reduce his salary just because he is underperforming. You either need his consent or a specific contractual right to do this.
In the absence of either, there is still a way you can implement this. The good news is that he has been employed by you for less than 2 years, which means he is not protected against unfair dismissal. This means you can terminate his employment for more or less any reasons and without following a fair procedure, as long as it is not discriminatory (I see no discriminatory issues here). So what you can do is to terminate his current contract and once that has terminated, you can re-engage him on the new contract with the lower salary, which he can either accept or reject and leave your employment.
This is your basic legal position. I have more detailed advice for you in terms of the steps you need to follow to implement this, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there is no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you