Six months is considered to average conception time for a healthy couple, i believe but could be wrong? I would suggest to your DH that he cuts back on his caffiene intake and you both start taking a multi vitamin..if he does'nt listen then maybe a trip to see a specialist is required to put the facts before him..sometimes we need to hear it from someone else...you would need to see a fertility specialist which costs $$$ and i can't remember if the sperm A costs anything or if its part of their service?
Goodluck maybe a weekend away here and there and doing some different things might help? often when you relax these things fall into place, easy said than done though!!!