If your horse has good hard "shiney" feet, nice coat, healthy weight, good hair growth, moves well (no joint issues), and has plenty of energy, then you do not need to add any supplements.
If you're only feeding hay, or hay with grain or feed below the recommended levels, then you should consider adding an all around vit/min supplement like Grand Vite or horsetech.com's High Point, just to make sure your horse is getting all of the vitamins and minerals he/she needs.
If you don't feed any grain, you can use a small amount of plain oats or beet pulp to mix in your vitamin, or you can get a ration balancing feed that you only feed 1-2 lbs of, like Triple Crown's 30% supplement or Buckeye's Grow 'n Win. Progressive makes some good ones too.
I do hear clicking in his joints, so I was thinking of putting him on a joint supplement. I have read however that joint supplements cannot be absorbed and are useless..... is this true? I was thinking of getting these: Majestys Flex Wafers - Statelinetack.com
They seem to help some horses but not others. I'd find one that you like and give it 60-days to see if it helps your horse. I have had good luck with Corta Flex and Acti-Flex 4000. Both are around $80 for a gallon, which lasts one horse just over four months, so quite affordable. That works out to about $0.65 a day, before you add shipping.