Sounds like your son needs more interactive activities. Playgroups are great. Try meetup.com to find one in your area.
And if your son is anything like my son, you will need to do more one on one activities to engage his brain and body. My son always liked to have books read to him or make up games with whatever toys we have on hand. He is now 3.5 and his current made up game is skeeball (like at Chuck E Cheese). I roll a ball on the floor and it hits him on the leg. He then falls down on the ground and makes the skeeball game sounds. He totally made that game up on his own :).
But at your sons age, I just made sure to take him to do some sort of activity where he could run and burn off energy at least once a day.
Thankfully we live in Poway, where there are a lot of indoor and outdoor kid activities to do. We go to the park a lot, walks around the block where I actually let him walk or ride his bike, the indoor play areas (My Kids Clubhouse or the Mall).
But also at 18 months I started my son in preschool 2 mornings a week. That really helped because they kept both his body and BRAIN active for 3 hours each morning. He goes to Incarnation Lutheran Preschool in Poway. It's a smaller school, but he loves it! He now goes there 3 mornings a week. And next summer my 18 month old daughter will start there at 2 mornings a week.
Other activities we did with my son at 18 months was parent and me soccer at the North County Soccer Park for one session. Then after that we did a class at the Little Gym for two sessions. Both of those let him burn energy, learn to play with other kids, and learn to listen to instruction from other adults besides his parents.
Now we do preschool 3 mornings a week and a bible study once a week and Sunday school at church. So he is engaged in something 5 mornings a week. Then on off days I make sure to take the kids out to the park, to meet with friends, or to one of the indoor play areas.