This may be a pollination issue. Many Avocado (Persea americana) can self-pollinate to some degree, but most will set a much better crop with cross pollination from another cultivar. With avocado, there are two pollination types. Cultivars that are Type A will have receptive stigmas (female) in the morning and will later release pollen (male) in the afternoon of the following day. Type B cultivars will show the opposite pattern. You will do best to plant at least one cultivar each of Type A and type B. If your tree is a seedling or unknown cultivar, observe the flowers to determine its Type. if you are looking at the flowers that closely you might also attempt artificial self-pollination to improve your yield. Oddly enough, in cooler climates, the timing of fertility in avocado flowers becomes less precise allowing for more self-pollination.
You cannot graft any fruit tree onto your Avocado, though you could conceivably graft on another Avocado cultivar to improve pollination. The remark about four fruits being grafted onto a maple (Acer) stump is also doubtlessly incorrect.
