I’m 6 years older than my twin sisters. Be prepared, particularly in the early years, for your son to be routinely ignored by other people, who are (I suppose naturally) fascinated by twins. Then there was a spell when my sisters were incredibly cute toddlers and it felt like the whole world wanted to tell my parents so every time we left the house. I was old enough to be annoyed by that.
Overall, though, the age gap had pros and cons. I was at school, already making my own friends, and as time went on, my own interests and activities. They started at the same school only just before I left, of course, and we went to different secondary schools, so I didn’t have to
put up with many comparisons, and they were too young to tag along with my friends and I.
I don’t recall ever feeling excluded from their relationship as they were never my peers, or all that interesting. I might have found it harder with a smaller age gap. That isn’t to say I didn’t love them dearly; I got into the worst trouble ever for beating up a boy who was bullying them!
On the down side, from about 14, I was expected to do a fair bit of childcare (my parents both worked and had an active social life) which was irritating and did nothing to make me want to spend more time with them.
And there was a lengthy spell from when I left home at 18 until they were 17 or 18, when we barely saw each other and had absolutely nothing in common at all.
As adults (we’re all in our sixties now), the relationships have fluctuated, inevitably - closer at some points than others. These days, we are friendly but I suppose I’d say not friends. We chat on the phone, WhatsApp, see each other four or five times a year, but don’t socialise together - part of that is geography, we don’t live near each other, partly that we still have few shared interests.
But emotionally we are close, and, sibling rivalry being real even for twins, I am closer to both of them than they are to each other.
And, having said all that, I don’t think any two families or any two relationships are the same, and so my experience likely won’t be much use to you!