Whatever you do, you need to stick with. If he knows that if he persists in getting up, crying, etc. that you will "rescue" him and pull him in to bed then HE has the upper hand and will just continue fighting sleep.
The bottom line is you need to start a NEW habit.
What about a cool new nightlight? We got this awesome light up turtle off of Amazon. It projects starts onto the ceiling and automatically shuts itself off. (Might be a "big boy" gift for staying in his room.) NOTE: It's not a toy for him, it stays on the floor or a dresser as the plastic shell is breakable plastic. Or change up his room so that it's something new and cool, but still familiar...like new sheets? A pillow? Cool pillow case? Friend to sleep with?
You can revise your bedtime routine so that he gets a bath, stories, maybe rocking in a chair until he's drowsy, then put him down in his crib.
If he cries, you can sit there in the chair, saying, "You're ok," but DON'T pick him up. It's still a CIO method, however you are not abandoning him.
If this is not possible, because you are alone and taking care of both kids, do a modified approach, leave, check on him after 1 min, then 2 mins, then 3 mins, as long as it takes until he passes out. Again, don't pick him up. Tell him he's ok and it's "night night time".
I realize none of these are "ideal" no fuss methods. However if he naps in his crib and is put there at night, and NOT "rescued" he will soon get the idea that it is HIS room where he sleeps.
Try to just focus on the sleep...not the drama of "does he feel abandoned", "does he envy the baby", etc. These are your projections. He just wants Mom...pure and simple. You are giving your son the gift of healthy sleep. And that is best for the family.
PS I don't know if the baby is co-sleeping or in a bassinet, but you may learn from your son that co-sleeping may not be a great idea. Kids quickly get used to whatever routines/habits we create for them. You may see a take 2 of this bedtime drama with the baby if you're co-sleeping now. Hopefully you can save yourself some grief later.
Best wishes