help has my partners sperm died

Me and my partner are trying for a baby I have had 4 kids from a previous relationship me and my partner have been together over 2 years and we have had unprotected sex every time but I don't seem to be falling pregnant ?? :( is there anything we can do to boost my chances or is there anything he can do! Also he has said when he was younger he had been kicked in the private parts many times and previous relationships he has had they have not fallen pregnant? Has his sperm died