solicitors will always advise you the most risk free way to go about things. they dont take risks. its not what you ask their advice for.
i think i would probably give discuss it with him, and if needed to give him a warning about dismissal, then get rid of him as you would at any other time.
he probably won't take you to court over it, but sometimes i think you need to show that you just won't stand for nonsense like this.
how long's he been working for you for anyhow?