OP: "I know how lucky I am but don't think I should be made to feel guilty about wanting a sibling for my son. He hopefully will have friends but its not like having your own brother or sister especially when most other kids do"
He as entitled to change his mind, for whatever reasons, as you would be. Who is making you feel guilty - you or your DH? It certainly isn't your young child, who has no idea whether they would like a sibling, or whether they'd got on with a sibling.
I can understand maternal urges - which do not have to be acted upon - but this seems very much about some notion you have that only children can't be happy. That's not the same thing and not, in itself, reason to have another child. MILLIONS of people around the world are only children. Do some, in later life, wish they had a sibling? Yes, some. Do some, in later life, go into fits of depression and become incredibly unhappy because they are an only child? Yes, some perhaps but not a huge number, I suspect.
You do know that just as many people don't get on with their siblings either as children or as adults later in life? As an only child myself, I do get annoyed when people start pulling this shit about "my poor baby is an only child" because it suggests that only children are somehow not as good, or should be pitied in some way. Fuck that.
I honestly think you need to consider some therapy to find a way to get through this, because you have a good husband (I assume) and a healthy happy child. Lots of people don't even have that.