It doesn't matter if it's old or fresh. His body is constantly creating and reabsorbing semen. However, having less sex can give sperm a chance to build up in his system. "If your partner's sperm count is low or marginal, you may be advised to have intercourse every other day to allow his sperm count to build up. Your doctor may even recommend that your partner refrain from ejaculating during the days leading up to your most fertile period."
But he should not go more than a few days without ejaculating. "Studies have shown that abstaining from sex for more than seven days can decrease the male partner's fertility. Any gain in sperm counts from lack of use is more than offset by the increased number of aged sperm cells with lower fertilization potential."
(diybaby.com)
Good luck and happy baby making!