The first thing I would say to do is to give your old guy some energy-giving hard feed such as corn, oats and sugar beet and possibly pony nuts/pellets. Then, in between your classes, tie him up with a hay net and give him his feed. Leave him alone for an hour or so then warm him up for around 15 minutes really getting him going forward from your leg. Use spurs (only if you have a secure lower leg and have been taught how to use them though) and carry a crop if need be. Be really enthusiastic 'come on boy, lets go have some fun!' just as you enter the arena. Concentrate on going at a steady speed and clearing the jumps, on the straight bits you can blow him out but make sure to collect his stride coming into a jump or a corner. Don't jump too many practice fences, 2-3 should be fine, you don't want to wear your guy out!
The bump on his head shouldn't matter, but make sure you are both turned out to perfection to catch the judges eye. A show browband with glitter and a matching tie are good but make sure they are acceptable at your show. Your saddle blanket should be shaped to fit the saddle for the showing classes but a thicker pad (or even two) is a good idea to protect your horse's back in the jumping. Wearing boots for the jumping (but NOT for the showing) is a must. For the in-hand showing, put your horse into his bridle and lead him with a chain and leather showing lead or the reins of the bridle, it looks so much neater than a halter. He should be bathed the night before and plaited and you both must look perfect! It is hard to get a grey looking shiny, but a good aplication of show sheen (not wear the saddle goes though) would not go amiss.
Good luck and let us know how it goes!