Hi! Actually the cutoff for IUI is usually 10 million motile sperm PRE-wash, so your DH's is pretty good, as long as the motility is ok. My DH's latest is only 12 million/ml but with a total of 48 million sperm altogether and 24 million motile sperm, and my doctor said we are good candidates for IUI.
Try not to worry

Sperm counts can fluctuate A LOT from day to day and month to month. I would try to have your DH take some vitamins to improve things, but it sounds like IUI is a great option for you. I have seen a lot of people have success with IUI with numbers like that. I really hope the first one works for you! We are going to be doing one at the end of January.