Do you have any expansion packs? If you have Hot Date, you can send your sim downtown. If you have Unleashed, you can send him to a community lot. If you have Makin' Magic, you can send him to Magic Town.
All of these are places where sims gather. There won't be many sims the moment your sim gets there, but they'll start arriving, and by the time you've been there for awhile, there will be several. You can talk with lots of different people.
You can edit all of these lots in build mode before your sim gets there and make sure there are things that sims like to do together. So you can add chess sets (with comfy chairs!) or televisions and sofas. After your sim has talked with someone for awhile, you can have him play chess or watch TV, then click on a sim he's talked to, and "Join" will show up as an option.
In the base game, you can talk to anybody and have a good conversation with them. In Hot Date, interests were introduced. The problem is that many of the sims from before Hot Date were created without any interests, so they'll hate EVERY TOPIC. But there is a solution for this! Play whichever family you're interested in, take them off of the lot -- to downtown or a community lot or something like that, and they'll be given interests.
If you have Hot Date or one of the later expansion packs, your sim will only have good conversations with someone if he talks about an interest that the other sim also likes. I used to keep huge lists of all the interests of every sim in town, so I'd know who to talk with! :-) Nowadays, I use Hedda's superscuplture from Some Sim Things, which lets you give your sim an interest. I go around the neighborhood and give every sim the SAME set of five interests.
Child sims have a bunch of child-only interests -- like Toys and School -- but they also have four interests that adults can also have: Weather, Sports, Music, and Outdoors. So I give every sim those four interests -- including the children -- and the adults get one more that they all have in common. Having common interests makes conversations a lot more like the days before Hot Date was introduced, when your sim could talk to people and just enjoy conversing.
Sims don't enjoy doing the same social interaction over and over again, so you need to mix things up. Talk, Talk, Entertain, Compliment, Talk, Talk, Entertain, Compliment works much better than talking three times in a row or entertaining twice in a row.
I hope this helps you get your sim's Social motive up!
"You must be the change you want to see in the world." -- Gandhi