A good picture would be helpful because tree ID can make a difference in the advice you get. We likes pictures and we likes tree ID contests.
Until then, here's some generic advice.
Busting off dead branches doesn't usually trigger new growth. It can increase light to inner branches, minimize places for diseases to start or spread, reduce humidity and the chances for fungal infections, minimize insect problems, and improve safety. Good for tree health, but not necessarily a trigger for new growth. Taking off live branches can trigger new growth, but you don't usually see that in evergreens.
Most trees get the majority of their water and nutrients from the roots that are relatively close to the surface. Push the water a foot into the ground and you may be below the root zone and wasting most of it. (Capillary action is when the water is pulled up through the soil by the attraction between water molecules. Deep watering helps here because there needs to be a continuous link between molecules. Once the link is broken, capillary action stops.)
Your clay soil holds more water and nutrients than the sandy soil. You don't need a "deep waterer" as much as you need a "slow waterer". If the water runs off (more likely on the clay than the sand), then your grandpa is right, the tree's not getting it.
Take a step back and look at the tree overall. Trees don't live forever and trees in captivity don't usually live as long as trees in the wild. Leaves should be a healthy green, crown full, bark uniform. Dead branches are a sign of aging (including natural self-pruning), disease or injury.
To keep the tree healthy, prune the dead wood (do it right, don't just bust 'em off). Water it slowly, test the soil to see if you need fertilizer, mulch the base, don't use string trimmers around it, and don't compact the soil.
