@Inneedofsomehelp11 You will be thankful that you have only one child whenever you book a flight or holiday, sign up for an after school club, you need to pay for tutoring for the 11+, they fail the 11+ and you think you can just about stretch to pay for private school, you pay for musical instrument lessons/gymnastics/football/karting/theatre society, you pay for driving lessons, you buy him a car, you pay for university accommodation, you help him with a deposit for a house, and you die knowing that he will inherit a house that he will own outright thus removing one of the biggest financial stresses young people today have.
All of those things get harder when there are two children, and harder again when there are three. Your gift to your son is love, wisdom, opportunity, and financial security.
Just ensure that your son has a wide social circle and he will thrive. He is fortunate to have extended family that he is in contact with, so why not think of his cousins as virtual siblings?
One note: IVF is unbelievably tough emotionally, mentally, and financially, and very many relationships do not survive the process. If you have concerns over your mental health, I would absolutely avoid this.