Your budgie is probably molting. During a molt, a budgie will lose many of its feathers and grow them back in. The spikes you see on top of his head are called “pin feathers.” These are new feathers growing in. New feathers first grow in inside a shaft that has a blood supply. As the new feather forms inside the shaft, the blood supply recedes and the feather starts to open from the top down. A budgie helps these new feathers to open by removing the old shafts from the feathers when he preens. The molt should last about a month or two. Often times the pin feathers on the head may accumulate and remain a little longer (see photo) because your budgie can’t preen his head very easily, unless he has a budgie buddy who is willing to do this for him. If your pet budgie allows you to scratch his head for him, you can help him open the new feathers on his head and cheeks by gently scratching or rolling the shafts with your finger nails or finger tips. Avoid any that you can see still have blood in them, as these are a bit painful to your budgie when disturbed.
If your budgie has a very heavy molt (losing lots of feathers, unkempt looking, lots of pin feathers) for longer than two months it may be cause for concern and you should call an avian veterinarian for advice. You can use http://aav.org/search/index.php to help you look up an avian vet in your area. But if he’s just having a normal molt, you don’t need to worry, every budgie goes through a molt once at about 3 to 4 months of age, and then about once a year.