Hello A.,
Include William in everything you do for Elliott!! As much as possible anyway.
You should be the one taking William to the park! There is no reason why you shouldnt be doing the things with him that you used to do. Have his grandmas watch the baby sleeping while you take him to the park. Or, better yet, have them come with you and Elliot and William to the park, so you can play with William while they keep an eye on Elliot. The air will be good for him! Just stick to your regular routine before Elliott came.
My oldest daughters are 20 months apart. Everywhere I went, my first daughter went with me. My mother also told her that the baby in my belly was HER baby and that she would have to help me to take care of her. She did everything but feed her, I breastfed (but she did sit on the chair with me; every time), and actually change her diaper and dress her! She brought me diapers/wipes, clothes she wanted her baby to wear. She decorated the baby's dresser, helped to make her crib up, pushed the swing when she was in it, helped wash, rinse and dry baby. Pretty much everything!
I dont know, maybe its different with boys, but my girls grew up to be the best of friends, and still are at 18 and 16.
Also, spend time with William while Elliott is sleeping! If you do arts and crafts, ask him to make something first for you, then for the baby, if he wants to. Ask him if he would like to pick out Elliot's clothes for the day/night, which blankets should he use? Let him have an opinion when it comes to the new baby. And, while he's sleeping, do something for him and say its from Elliot because he loves him and wants to thank him for being a good big brother.
You'll be surprised how much he will change once he realizes that he is still part of the family and you still love him just as much. No matter how many times you say this to him, he's only 2 1/2 and wont understand these words, actions are what will show him that he's not being left out, or left with grandma!
After a couple days, or maybe weeks, you should see a difference in William. And, dont forget that if he gets too out of control, he does still need to be punished, for hitting. Talking to him once or twice, (Please dont do that, it hurts me and hurts my feelings, its a naughty thing to do, and if you keep doing it, I will have to punish you, time out) Remember not to say that you are afraid that he will hurt the baby, this will cause resentment. Do not include Elliott when punishing his naughty behavoir. He'll begin to think its the baby's fault he's in trouble.
And, I'm sorry, I know this is long, my responses usually are. Do not tell William you can not do something for him because of Elliott. Ask him to help you so you can get something done a little faster, so you can do something he wants you to do. I know, it doesnt go faster, but it is a way to include him, and take his mind off what he wants for just a couple more minutes, and you can finish what your doing with Elliott; as family.
Good luck.