Worried re First Born

I have a 9 1/2 month old boy, who means the world to me. He is a really good baby and he is with me nearly 24 hours a day apart from when dad is around. ( i have no family nearby although he does spend the odd hour or so with my friend I am going back to work for a few hours a week and he will go to childminder which is fine. I am now pregnant with twins and my son will only be about 14 months when they are born. I am worried how he is going to react to this as I really dont want him to feel left out. He does spend time with other babies and seems OK, and I obviously know it is important to keep some alone time just for him and mummy or him and daddy, ie Any advice!