I would give him time and tell the kids he needs time to adjust (a good way to help them learn patience I know I have a 4 year old
) I am sure once he adjusts and get's used to the environment he will play and they will all end up being best buds. I will tell you he might be buds with your older children more than younger. I know my Georgie is still nervous around my 4 year which he has every right to be our son is 60lbs and Georgie is 4lbs. Georgie was also the baby for 2 1/2 years before I had my son.
Georgie still watches from afar and really we are ok with that it's better for all involved given his size. However, he does play with his little big skin brother more now then he ever has so we are seeing progress
Isaiah will ask to hold Georgie and sometimes we oblige and let him when he is sitting down right next to us. It's to dangerous for him to hold him while standing up or walking.
Best of luck I am sure it will all work out.