Question a key remote employees commitment without risking the relationship

I own a completely remote company, 12 full time employees in total split in several countries in 2 timezones. We are very relaxed in terms of timesheets and working hours, as I don't beliefe in 9-5 or looking over someones shoulders. For most of my team I have a good overview on what they do, can estimate how much work they get done and how much of their full time commitment they actually put in.

However, there is one fairly senior guy who is tasked to to run a part of the team and parts of operations, and by the looks of it he is only working 20-25 hours. I'd like to have a talk with him to assess his commitment and figure out if he does other stuff on the side, without crushing the relationship we have... I cannot know if he only works half the time or not, without asking half the team (which is a no-no for me) or risking a broken relationship with him if i ask bluntly.

I am not very happy with the work he does, but certainly beliefe he could do a great job... our company is growing quickly, closing a 5m round in the next weeks, he has a vesting plan so im not really sure what else i could offer him to move him towards full commitment.

How can i figure out if he just works on stuff that is not completely visible to me or if he's just cashing in with little effort?