You can install the client on as many computers as you want.
If you want to play together with your son, however, each of you must have your own account (meaning you each must have a unique username as password). If you try and log on with the same account, one of you will get kicked off.
"If A equals success, then the formula is: A=X+Y+Z. X is work. Y is play. Z is keep your mouth shut." --Albert Einstein