A 7 year old should not have a TV or a computer in his room. It's totally tempting to hole up in there and be anti-social. TV is a family thing and a public thing - you need to know what he's watching, and you can participate in it that way.
It's normal for 7 year olds (or any older sibling) to retire from the baby chaos that surrounds any newborn/infant. How long has his stepfather been in the picture? Are they still getting into a routine, and now there's a baby in the mix? Is his father in the picture? If so, did you move farther away and now visitation is more difficult? It's not an excuse for him to check out, but it can be an explanation. Get to the bottom of it - you have to build a family here.
Stop "inviting" him to participate because that means he can decline. It's better to say, "Grab your coat, we're heading out" and go do something fun. What's in your city? A zoo or children's museum, a children's wing of a library, a nature trail? Go learn about your new area! Do some things as a family that are more for his age, and just take the baby along. Find out if there's a set of passes at the library that allow you to attend events/attractions for free or a reduced cost. Are you a member of AAA? They often have coupons and suggestions and a guidebook to area venues.
Has he made friends at his new school? Can you invite one of them to come along with you? Can you schedule play dates? The happier he is with his life and his surroundings, the happier he will be with you.
Call the high school or a local college, or the local agency that runs a Red Cross babysitting course and see if you can find a babysitter (perhaps a "graduate" from a few years ago since you have an infant). Have a new sitter come once or twice while you are home, and then venture out leaving the baby at home, so you can do things with just him and his stepfather, or leave stepdad home with the baby and take off for a mom-kid date.
But it's also normal for kids to become a little more independent and do things with friends. You may also be feeling a little nostalgic about your times with just him, as you are also adjusting to having a baby who, no matter how much you love her, cramps your style and kind of controls the schedule. See if you can break out of that.