The only reason that this would be an issue would be if he's saved it up while not paying his fair share of joint expenses or otherwise claiming to be skint, and expecting you to cover the gap.
Re: the furlough, you say it's an ISA, if it's a stocks and shares ISA, the value could have dropped this year, and unless you have no alternative, it's a bad idea to take the money out.
You say he earns well, this money could just be money that's built up naturally over time, because he's earned more than he needs or wants to spend, especially if his costs have gone down after he moved in with you due to sharing expenses etc. He might not be saving it for anything in particular, it's just spare money that's not been spent.
We're sort of in this position, but not quite as much. I have £30k in premium bonds that DP doesn't know about - I would tell him if he asked.
That money has just built up over a good few years because I'm a saver and he's a spender and I naturally spend less than he does, plus we have very low outgoings and have a fairly decent income.
We both get the same amount of personal spending money each month and he blows through it all on fuck knows what whereas I usually have over half of it left over so I just put it into premium bonds or otherwise save it. We're talking about the result of me naturally spending £50 a week less than him on personal stuff for 10 years. But that's OK, because I'm a woman 
.