If he doesn't raise his paw, repeat the process of saying shake, grabbing his paw, shaking it and giving a reward. Do that often enough and he will know what it is that you want him to do when you say shake. Dogs hear the pitch and loudness of the words you say more than the meaning of the words. It helps to say each command in a more forceful tone than the dog is accustomed to hearing. Be consistent in how forcefully you pronounce the commands.
Body language can be important.
A gesture such as putting your hand out when saying shake can be much easier to learn than just the word shake alone. Likewise lowering your hand when saying lay down will be learned more quickly. Try to make each gesture much different from the gestures you use for other commands. Other tricks you can teach your dog are lie down, beg, come here and fetch. Some more difficult ones are stay, play dead, roll over, heel and speak.