Good suggestions here.
Adding this: the priority, should be your child.
If at ANY time... you see behavioral problems, your child being stressed, or not understanding things.... if/when you do tell your son... then you have to address that.... not just punish/scold a child for NORMAL emotion based inability or confusion to deal with something or not.
Kids this age, do not have, 'coping skills'.... for dealing with things. Many adults don't even have coping-skills for emotion based issues... so, don't expect a 'certain' response from your child, whether that be understanding or not-understanding... because, even adults have inability for that.
Watch for the 'expectations' you have upon your child... they have tiny shoulders that either can or cannot 'carry' our adult burdens.
At this age, they don't even know what a 'relationship' is and all the abstractness of it.....
Don't accelerate something, unless you know for sure... how to deal with it... and think of your child....
Next: the logic that "if a boy loves a boy and a girl loves a girl they can be together..." may not make sense to a child. This young. Even adults can't understand that.... so again, watch for what you 'expect' of your child's understanding....
AND... if you do tell your son... then, you will have to deal with whether or not... HE will tell his school friends/parents/Teachers about it. Too. Or you have to have secrecy about it all... which can be a BIG 'stress' or burden for a child's tiny shoulders.... and how will you deal with it... if your child just innocently tells other people.... or what if he gets teased or people make comments to him about it???? Will he keep it all bottled up, or tell you? How will it affect HIM????
The gist is: a child cannot be 'responsible' for their parent's lives nor keeping it a secret or not... or if it is out in the open... they cannot be 'responsible' for the ramifications of it... nor know how to deal with it...
keep expectations of him, age-appropriate.... and you must gauge him closely... and his behavioral/emotional well-being. THAT should be the priority....
Next: How will his Dad respond to this? IF you tell your son. Or does your son have to keep it a 'secret' from his own Dad??? That is a heavy burden for a child. Even adults can't deal with things like that. How will your son be able to juggle it all and deal with it????
IF your son is expected to keep it all a secret... then, that is like 'lying' to his own Dad. Is that fair to expect from your son???? A young child....
AND, how will your Ex/your son's Dad.... deal with it and you and your son????
Then to extrapolate on it all: HOW will your son deal with it, in relation to other family members/grandparents/relatives/other friends... WILL he be able to tell them too? Or not? How will you instruct him on that??? And, can he deal with it??? Can you? Will you tell your relatives/family/your own parents about it???? If you don't know nor have the answers to these issues yourself... then, you cannot expect your Son... to know how to manage it all, either.
**One of my siblings is gay... I have HAD to keep it to myself since I was a child and knew. THAT, for me, although I am an adult now... is a BIG BURDEN... to have to keep, to myself. And I resented it and it caused problems. It is not "my" problem... but I inherited that problem. I should not have to be 'responsible' for another persons 'secret' life. And it entails having to 'lie' to my own family/other people... when they ask "How come your sibling isn't married?" How come your sibling has no children or significant other?" "How come.... your sibling has no one?" and on and on and on. It is a big, complicated, thing. And I really... don't like to have it HANGING over my head. ALL because, this sibling chooses to keep it a secret and not want "me" to tell anyone... and even if other people knew, THEN I would have to deal with the "how come?" questions too... which is equally a problem.
I know each circumstance is different... but, just my scenario....
Next: Your Significant Other... should not pressure you about it. This is YOUR child. And that is your priority.