Best friends feet

I don't know if this might be a bit of a niche experience. But if anyone has had a similar experience and maybe has some advice.

I'll start by saying we've been best mates for years and he's 100% straight and very comfortable in his sexuality. He knows I like feet just doesn't know how much I like feet haha. It started over a year ago we were at a 5 day festival a lot of walking happened over them days. On the last day we were at camp and he asked if I'd rub his feet for him. I told him no as we'd been at a festival for 5 days without a shower. Even I have my limits haha. But I told him when we got back and he'd had a shower I would happily do it for him. It was great haha ended rubbing for feet for about 2 hours. After he made a comment saying how good the foot massage was and that he'd be asking for them a bit more in the future. Which I said id be happy to do for him. And he did happen a few times. But early this year it came I found out he also has like feet obviously women's feet though and he also loves having his feet played with so after that conversation had been. If we were just chilling and watching TV most of the time he'd have his feet in my lap and I'd be playing with or rubbing his feet. But nothing more has ever happened. But my issue now is where I'm so close to his feet more often now I've realized how nice his feet are and I'm growing a bit more obsessed with them haha. So it's becoming a lot harder for me when I'm around his feet to keep my cool. I don't want this to stop. But I don't know how to keep going without losing control a bit and cross a line that I don't want to cross. I've had a conversation with him recently just double checking with him that he's comfortable with what's happening and he's confirmed that he is and if he ever isn't he will say something. So I'm now just trying to find a way to be able to continue being around his feet while not making it weird but losing control and doing something silly