This varies so much from one of us to the next. If he's begun puberty he'll begin to notice alot of changes. However, some of us continue growing in the penis department until we are in our late teens so it is very hard to tell. There is a wealth of information on the web about this topic, some sites provide charts and age/size information. I'd suggest doing some search using the term penis and size, may also get somewhere by using masturbation etc. Be careful thought, this may take you to porn related information and that may not be helpful at all!