Celina I think your fi should wear a white shirt with the ivory embroidery and the natural linen pants. IMO a white shirt with white linen pants is too much white. Im sure it would look good, i'm just not a big fan of all that white KWIM.
I think he would stand out fine from the guys since the embroidery would be different.
God i've changed colors so many times now, but the one color that is fore sure is turqoise. my girls dresses are turqoise (i only have 2 bms so they have same dress). For guys my fi is wearing a long sleeve shirt with turqoise embroidery. The gms are wearing short sleeved white linen shirt with the turqoise embroidery. all are wearing natural linen pants. i thought the sleeves being different would be enough. It's so hard to figure out what will good in pics and what is enough to make your fi stand out without seeing all together.
And i do like how you are having the Moh and BestMan match and than the bms and gms match. I think that will look really nice.