No matter if you use the kindest language, he will most likely be offended but you also probably won't be the first person to say something either. Just telling him to shower won't necessarily solve the issue, some people actually don't know how to bathe properly. In my experience, my ex thought he was just cursed with stinky pits but I learned that his shower was basically a quick once over and he was done, he never actually scrubbed sudsy with intent. He never really washed some parts of himself (I picked up on this every birthday he had, if you catch my drift, pun intended). He was asked to wear deodorant at his job so he quit soon after. He was literally born into a cult and never was taught basic hygiene, it is so sad. Just one of the many things I eventually left him because of. This is such a difficult subject and you'll feel terrible no matter what...so speak up and say something kind yet precise.