The 3 biggies are:
St Johns Wort - either Kira or Perika brands.
Fish Oil
SAMe
Also could consider deprenyl 5mg ED, although it's not strictly OTC.
Seeing a doctor depends IMO on it depends on how bad it is for your dad. A psychiatrist is just going to prescribe something like Paxil, Wellbutrin, or Effexor. These have a less than 50% success rate and have lots of side-effects. So in this case, try the natural route first.
Of course if it is severe, then yes, he should go see a doctor. But if it's mild, then honestly seeing a dr and getting prescribed the pharmacy will do more harm than good.
A lot of it could be psychological too. Midlife crisis and all that. In that case talk therapy helps and if his insurance covers a psychologist, then he should check that route out.