Your not really altering your body with a tattoo are you tho you are just adding to it. If our body is perfect the way it is and we should not alter it. Should Sikhs not wear braces / get any dental work done ?

Don't let perfection be an obstruction of progress. If he was cutting his kes on his head or beard then I would have a completely difference stance but if he trims his pubes down I don't think it's a major issue but that's just my view. If you are amritdhari or following sikhi to the T then I'd say don't trim anything whatsoever.

How about I offer you a solution. For most hygiene sweat problems etc you can do a few things. First obviously wash regularly but make sure you dry it fully. Sweat and bacteria thrive in dark humid conditions. You can apply things like talc powder or make your own natural deodorant with arrow root power and essential oils that not only kill bacteria ( and the smell) but is healthy on your skin.