scully29 I won't tell you whether this is the right time to adopt or whether you should make decisions on another child in the family based on what your son thinks.
I will tell you my own experience. My dd was desperate for a sibling and really wanted another girl or a baby. All along we said that the child may be a boy, and may be about three. And when we adopted he was a boy of three (I had read that at the time, 7 years ago, in our area the most commonly adopted 'demographic' was boys aged 3!).
Anyway, from the start my dd found it very difficult being a sibling to her brother. She was very jealous of our time being given to him.
A few years later she was diagnosed as being on the autistic spectrum and that does explain some of her (rather bad) behaviour.
Our son loved his new sister from the word go but over the last few years her bad behavior and his own bad behavior has made their relationship quite difficult.
I do not regret adopting our son and he is a wonderful boy. We did not want our dd to be an only child and although at this time they do not get on, they do have each other and I hope that one day their relationship will be much better.
My advice to you is to do what is right for you as a family as a whole.
I'd also say that one thing to be very conscious of is not to pass on any items from your two children to your new child unless they expressly wish you to do that. This is very important. In the normal run of family life it is quite common to pass on clothes and toys etc but I do think that this can cause a lot of anger/jealousy. etc.
Either buy your new child new items or even second hand items but do not pass on items like toys or clothes (oven furniture) unless the child is genuinely relinquishing them.
It may help to get to the bottom of why your son is upset, and to reassure him of your love and of his place in the family.
It also may be of use to consider that he may feel his place in the family is and if somehow he feels it is being usurped. Is he the youngest? He will not be the youngest anymore. But then that would be the case if you had another birth child.
Is he the only boy? It is possible you could look for a match with a girl to ensure he stays the only boy or there is not another boy in the family. But then again if you did have another birth child they could be a boy.
Good luck, whatever happens, I hope it all goes well. 