Natural sweat stain removal?

Hi, I don't use anything tested on animals or harsh cleaners that hurt the environment (eg bleach) My boyfriend sweats profusely & im having trouble with his stains in his armpits, they are yellow & actually make the fabric stiff & ruins shirts after only 2-3 wears. I have soaked them & used natural wash booster... show more Hi, I don't use anything tested on animals or harsh cleaners that hurt the environment (eg bleach) My boyfriend sweats profusely & im having trouble with his stains in his armpits, they are yellow & actually make the fabric stiff & ruins shirts after only 2-3 wears. I have soaked them & used natural wash booster but i cant get rid of them, any ideas? His polo shirts for work are white & it really effects his self esteem.
Update: FYI- I have actually used bleach on them in the past but that didn't work at all either.

He has used every deoderant known to man, nothing works at all!!!
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