I heard sex every two days and do it when he feels like it (if you like) as that way the sp*rm, or troops as some refer to them, will be the liveliest! So if he is a morning person, go for it in the morning.
Try and be as s*xy as you can so it does not feel like a chore! That way you will get his best results and he (and you) will be less likely to lose interest!
Also, not sure how correct this is but can't hurt, read up on it for things to help, ask dear old Auntie Google! E.g. you could prop your legs up against the wall after sx, (many will say this is not necessary and I have no idea but you could give it a go!), also try and have an orgsm after your DH has done his bit as this may get things moving in the right direction!
Avoid any lubricants, these may kill sp*rm, or at least check, I have even hear saliva can!
After s*x not too much movement of any kind, as this can disrupt the troops! For a few minutes at least.
Anyway, these are all things I have heard.
Our only child was conceived via assisted conception (IUI) and of course that is something you could explore, using IUI, if necessary. All the very best.