It always seems to be the way, one partner is keener than the other. I wish mine would stop pestering me for sex, I'm just not interested in the slightest and would be happy never to do it again.
Perhaps your Partner is stressed about something, which means sex isn't a priority. He maybe genuinely tired or feeling down.
You've tried talking to him, but unfortunately men tend not to be too open about their feelings. Perhaps you should back off a bit, give him a bit of time and see if things improve, if not then you may need to seek professional help.