This is just pure guesswork since you gave no details, but he may reside in a house or work where there is a lot of cooking with spices - as well as garlic and onion, although believe it or not, you don't need onion to produce onion scent. (Sometimes the happenstance mixture of other scents produces a scent not dissimilar to onion or garlic.) This attaches to his clothes - particularly his t-shirts - and this rubs off on his skin. You notice it more acutely on the nipples because that skin is more fatty and somewhat absorbant and therefore you notice the scent more clearly. How can you tell him? Try indirection. "Honey, I've noticed an onion scent on your clothes lately when you come in. Have you been to a restaurant or something? " Then add after he looks at you confused, something like, "I've noticed it even on your skin a little the last two or so times we were together." Best I can recommend. (By the way, not sure if you are into this, but a man's cum will tend to pick up a slight flavoring depend on what they have been eating. If something sweet, it will tend to be sweet. If bitter, then bitter. This may be a way of deducing what is going on. Again, if you are into oral sex and swallowing - or at least taking into your mouth - your guy's cum.)