My 6 year old asks about a sibling occasionally. She likes the idea of it, not so much the reality of babies, if her interactions with her cousin is anything to go by. Sadly for us it just isn't on the cards, our eldest daughter was lost in pregnancy after 11 years of unexplained infertility, she also had a chromosome deletion that meant a lot of tests but was found to be 'just one of those things'. I found my second pregnancy to be very hard, both mentally and physically as I was constantly worried the same thing would happen again, which combined with PTSD and other health complications meant pregnancy was a thoroughly miserable time. We decided not to test our luck any further and drew a line under it not long after DD was born. DH has had the snip. If the subject comes up I simply tell her that daddy has had a special operation and mummy is too old so we can't have anymore babies. Then she asks for a dog.
I am one of two, I get on with my brother, now, I wouldn't change things. I love him very much. My mum is an only and my dad is 1 of 3, both ended up being the primary carers for their parents, mum because there was no one else until I took over due to her ill health and my dad because one brother sadly died and the other is an arse who scarpered the minute he could see that his mother needed more help, leaving all of it to my dad, showed up for his inheritance though.
As you can see there are no guarantees, you should have a second child because you both want one not to give your child a sibling. And if one does not want another then that choice takes precedence. No one should be forced to have a child they don't want.
Plenty of kids are only children and are just fine. Your husband may change his mind, 3 is a pretty full on age, he may find four or five easier and revisit his decision. He may not. Whatever the outcome you have an existing child that must come first.