Is there anything that can negate the effects of his marijuana use on conception?

No. The hypospermia, dysspermia and hypotestosteronism from mj may or may not be reversible. Nothing will mitigate it while using. May or may not improve on cessation of cannabis. See http://www.Health.Gov.Au/internet/main/publishing.Nsf/content/health-pubs-drug-cannab2-ch65.Htm.
Stopping. If you are refering to the association between cannabis use and male fertility, discontinuing cannabis use should restore one's sperm counts to their pre-cannabis levels. I know of nothing a person can take or do for this excpet to stop smoking the cannabis.
Sorry. Tell him to grow up and quit weed. If he can't then don't have children!