Hi J.,
This is what worked for me: Give your son lots of playdates with friends, let him feel he is is control of his choices- sign him up for classes- sports, art, music, things that he wants to do, always what he wants - the YMCA is a great place to start- it's inexpensive and so much fun for a boy! I know this means you have to take the baby and toddler with you, but this will only last a couple of years... and then your son will have wonderful memories of feeling happy, fulfilled with you and his siblings for the rest of his life! Your energy does have to be shared with all the children, give it all you've got. You'll be glad you did.
Best of luck!
L.