Hi OP
After the birth of our first child (he’s now 4 years) both DH and I agreed to only have one child. This had sort of been discussed and agreed upon whilst I’d been pregnant anyway really.
Anyhow - when DS was about 14 months old I started to want another and one of the big driving factors was that I wanted him to have a sibling. I used to watch other children playing and having fun with their siblings and knowing there was that special bond made me feel incredibly sad at the prospect of never allowing my son to have that. I didn’t feel guilty though, just sad at what he’d be missing out on.
I spoke to DH and he categorically said no to having another child. As the months past though my feelings grew stronger and I felt upset at the thought of us as parents, forcing DS to be an only child, because we (well DH) had made that choice.
I spoke to DH again, and again and again and there were lots of tears and eventually when DS1 was just under 2 years old (so about 10 months after me first talking to DH) he agreed we could TTC for another baby.
It took a long time to get pregnant and ultimately there is a 3.5 year age gap which in my opinion is still a little too big. We had another boy, he’s just about to turn one, and the relationship between them is amazing but I’m well aware that because of the age gap, the potential for their relationship to go awry when they’re older is a real one.
I think if it whilst TTC the potential age gap between DS1 and another baby continued to grow I would have stopped TTC. As I said the main driving force behind having another baby was to give my son a brother/sister they could form a relationship with, but if the potential age gap had gotten bigger than it would seem pretty pointless as it was likely our first son wouldn’t have any inetetrst in a much younger sibling as the years passed.
I know siblings with 5-6+ year age gaps and although the love/bond is there in the early years it eventually fades off as the two children eventually end up leading separate lives from each other as opposed to wanting to spend time together, and this separation then continues into adulthood. This is not true for all large age gaps obviously but it’s just my perception of what I see occurring around me.
I know you’re upset because you wanted a second child anyway, but I would try not to let your child’s sudden desire for a child cause a rift between you and your DH. Even if you did decide to TTC and you got pregnant straight away, your son would be 11ish when it was born and how would that actually benefit him?
When he’s 16 years old is he going to be telling his mates he doesn’t want to hang out with them because he’d much rather chill out with his 5 year old brother? I doubt it.
And when he’s planning on college/uni and the arrival of girlfriends I’m not sure how much time he’s going to have for his much younger sibling and when he hits adulthood and starts living an independent life, how much in common will he have with a 10 year old sibling? How much in common will they have?
Like I said, this isn’t the case for every big gap sibling relationships but they are certainly worth considering.
I think you should have a very frank discussion with your son and ask him exactly why he wants a sibling because wanting one just because his friends have one is a bit of a flimsy reason.
If it’s because he says he’s lonely then put the above scenarios to him. He’s too young to understand the complexities of what he’s asking of you but I know it must be hard to see him upset 
Please don’t feel guilty, afterall, when he’s flown the nest in ten years it won’t be him having to look after with the sibling he so desperately wanted, it will be you. I imagine he would give little thought to a much younger sibling in terms of wanting to spend time with them.
I think you need to find a way to address the upset you feel towards your husband but based on my own experiences I can completely understand why you feel the way you do.
However, don’t use your son’s request to make you feel worse about the situation or risk your marriage because the likelihood is that despite what your son is saying, in reality, providing him with a sibling probably won’t provide much of a benefit to him as the years go on.
Like I said, I do understand why you feel the way you do but don’t feel guilty towards your son just because your son wants a sibling, I’m sure it’s just a phase. Would you really want to go all the way back to square 1 again to make a 9 year old boy happy who this time next year may feel completely different?
I hope nothing I’ve said has offended or upset you or any other poster, I just think you need to think about the bigger picture and realise that you although you’re upset to see your son like this, having another baby for his sake in your current situation isn’t the right thing to do. Don’t be harsh on yourself OP, your son will be fine as an only child 