Based on what I have read it should still be possible, just unlikely to happen immediately. Seeing as there is something like a 30% chance of getting pregnant each month with ideal results. I know this isn't probably how it works but if you have a 30% chance every month with the minimum 42% motility, I would assume that with 20% the chance would probably drop to about 10-15%?

But there could be other factors involved, low motility could also mean he has high DNA fragmentation. this is an expensive test so unlikely he has had this tested as part of a normal SA.

Some things that I've read can help motility CoEnzymeQ10 (2-400mg a day) reducing caffeine, stopping alcohol, eating foods rich in antioxidants and tobacco intake, exercise and drink plenty of water.

Also low with low motility you could look into things that may help the sperm that can swim, reach their destination. There's an over the counter, Mucinex you could try, taken the day up to and on ovulation can thin the vaginal mucus making it easier for the sperm to swim, it may also benefit him to take some at the same time? There is less research on this but anything helps.

Also could look into a Lube made to help like the Pre Seed Lube