If I understand correctly, Elijah is your husband's son from a previous relationship, and you are the stepmom?
It sounds like Elijah hasn't had much discipline, but it's hard to tell whether it's that or just his age (toddlers often test their autonomy by spitting, hitting, etc.). Older children should be expected to behave more appropriately, but young toddlers do need to be taught proper behavior, too.
I would guess that your mother-in-law is feeling a bit jealous at this point. She's not only been Elijah's alternate caregiver, but she's not had to share her son with another woman, either. You didn't say what happened to Elijah's mother, so I can't comment on whether she was a thorn in his mother's side, as well.
It sounds like your mom-in-law has declared "war" on you! She's vying for the queen bee position, and unfortunately for you, your husband will have a great deal to say in this matter. I suggest you talk to him and if he's a strong man, he'll support you and let his mother know that unless she can work as a partner with the two of you in teaching Elijah proper manners and behavior, Elijah's visits to her home may be curtailed or stop entirely -at least, for awhile.
In fact, it might be a good idea to do just that. If you have recently married, then Elijah needs to get used to having a stepmother. He needs to bond with you as a primary caregiver, and Grandma needs to relinquish that role. It probably won't be easy for any of you.
If your husband is a weak man, he won't want to say anything to his mother, and he won't support you. In that case, you've got more problems than just his mother.... Ideally, this should have been discussed before you married into this family, but many people don't think of these things beforehand.
In my opinion, if Grandma can't or won't get with your program of discipline and instruction, then I'd say that Elijah doesn't go to Grandma's house for awhile, until you're sure his bad behavior has stopped. Then, resume visits when you can accompany him. If Elijah starts with the bad behavior, you cut the visit short, scoop Elijah up and take him home. Period. If Grandma starts with her "my house, my rules", ditto. You take Elijah and leave.
Might be best if hubby isn't with you, unless he can support you on this. As I said, it all depends on how strong or weak this guy is as far as standing up to his mother and wanting to raise his son to be a decent and respectful young man. Because if you don't do it now, it sure won't happen when the kid is older. You have to train kids when they're young.
Good luck!