If your DH can not see how much this is upsetting you and consuming your life (let alone not appreciate the damaging messages you are subconsciously sending to your DS that he 'isn't enough' for you), and will not pay for the desperately needed counselling, then why on earth would you consider having a second child with him?
You can see from all the posts here by people who have had one child by choice, or one child by circumstance, that 'for others' having one child can be the best thing ever, for the parents and the child. Their circumstances, when viewed from a spaceship, are no different to yours. Their child will have as enjoyable childhood, as many friends, have as many skills, have the same chances of a happy relationship as your DS. The only difference between those posters and you, is YOU. You are the one out of kilter with your extreme reaction to having one child.
The saddest words you've posted are where you say that your DH feels similar but he works away a lot so you have to "bear the brunt of it alone". Bear the brunt of what?? You have a healthy, gorgeous, charming 5 year old boy who you should be grabbing life by the balls with!
I'm going to be harsh here. I'm going to paint a potential scenario you're not going to like. You ain't having another child. In 60 years time when you are lying on your death bed in the nursing home, you're still going to have only one child visiting you. Accept that for a minute. Picture that for a minute. You could either have spent the previous 60 years of your life focussing on what you don't have (and have lived a shitty half-life, probably damaging your DS in the process, through only ever being emotionally half-available for him) or you could have spent the last 60 years of your life doing all the fantastic things that life can offer and loving and cherishing your DS as the centre of your world as he deserves. Life is so so bloody precious. I can't believe you are prepared to waste it.
You need to prioritise getting counselling above everything else you are doing at the moment. Book an appointment with a different GP today and demand it/put aside money each month from your housekeeping to save up for it/borrow money from family to pay for it.
You need help. You need to get that help.