You just described our twin 2 . Twin 2 has always been the quiet more reserved twin, she allows her sister to talk for her, always has done. She does however have hearing issues which we are waiting to fix which also accounts for some of her lack of speech, apart from the fact that its not uncommon for an older or more outgoing sibling to talk for the younger ones. Its worth getting the hearing tested periodically if he is still lagging behind his sibling, just to make sure its just a sibling thing and not missing some underlying problems.

Try not to compare them though, as although they are twins they are two separate human beings with their own personalities.

For instance, our Twin 2 had a slow start and does things in her own time when she is ready, she is quiet, cuddly and emotional, takes her time to take in everything around her. Twin 1 on the other hand is boisterous, loud and thinks she is just like her older brother! Twin 1 hit the ground running and was ready to take on the world, doing everything at least 6 weeks to 6 months before her sister. Twin 1 is a leader, Twin 2 is a follower. They may be twins but they are different in almost every way personality wise.