He may be coming towards the end of his working life, however, as he has recently produced piglets he may still have the ability, the only real way to find out is to mate them. More importantly he should not be too big as the young gilt has to support his weight during mating which can last upto half an hour.
Once he has mated with the gilt she should not come back into season, if after 21 days she does then maybe it's worth another go or try another boar.