Good Morning T.,
My suggestion is Patience with your child. His new situation is more difficult than we adults imagine it to be. Don't send him back to his grandparents' house as this will only prolong the adjustment to the new enviroment. As for laying with him, I strongly suggest you sit in a chair with him for some moments only. Talk about the changes and how difficult they are for everyone. Say that you are all adjusting and it takes time. Remind him to think of the positive points of this change such as having some step siblings to play with him daily; a step-father around to offer just one more person who loves him; his own room. Point out the positive aspects, disregard the negative side for now especially if it is only that he's not sleeping with his grandpa near. Talk about family gatherings that will occur inviting the grandparents over to grill out, or for a sit-down dinner, etc.
He will adjust in time. If there is no harmful reason for him to fear being in this home with a new dad and siblings, it will then be just a matter of time. I say this because I do not know you, your spouse, your step-children. And therefore do not know what goes on in this group of people who've been joined together. I'm basing my suggestions on the assumption of this being a loving relationship for all involved.
Relax, T.. In time all will be happy!
God Bless you and your new family!