Home nudists coming out into public
I totally agree with Tennstud about penis size - I have seen everything from pea size to knee length flaccid in naturist environments and nobody too the least bit of notice of any of them.
You can't help seeing them, of course, but the same is true of the nose on your face.
I'm betting that even if he's totally bare ass underneath that being wrapped in a towel would be an acceptablemeeting point for both worlds. He's still technically naked and yet still covered. That can give him time to adjust and get calmer about it - and this,, I will betcha , will happen a lot more faster than either of you (especially HIM) thinks. A few minutes of just sitting around and he's gonna see he's not the tiniest here nor the fattest there or the baldest or the hairiest or what the heck ever. He's gonna see that it's all in balance and that all the different shapes and sizes make a perfect blending of imperfections. He's then also likely to notice that, absent the threads, people to get more open and honest and altogether more "real" once they have made themselves "vulnerable" to the eyes of others. While generally specific to the manhood, I'd file all of that stuff under "insecurities about size" because you find it with waistlines or with women's breasts, etc etc. All you gotta do is be brave for just a couple of minutes and you'll see it ain't nothing but a thang.
If you suspect it's about getting a boner, I'd flat out ask him while following immediately in the next sentence before he answers that you get it, they probably will too, but regardless you're willing to make a game plan for how to help him into and out of pools or any such situation - sitting in front of him, being quick and handy with the extra towel, or whatever you might come up with.
This is so totally doable and so totally worth it.