Just be patient with your ds as he learns how to do things and works out which way is best for him. Using cutlery, scissors, pen/pencil etc. Learning to do things can be frustrating at the best of times, so just encourage him to try them left-handed and right-handed and think which way is easiest as he is doing them, then he can choose.
I am a leftie, along with ds4 (8). Dh and other sons are all right-handed.
My dm is a leftie too so I grew up with left-handed scissors. We now have ambidextrous scissors, potato peeler and bread knife. (Also have a right-handed bread knife, it is only sharp on one side, the other is sharp on both sides of the blade.)
I didn't realise until I was in high school that I could use different hands for different things. I write holding the pen in my left hand, but realised in PE in high school that I have better hand-eye coordination with my right hand. Started getting rounders holding the bat in my right hand, improved my tennis, badminton etc game holding rackets etc in my right hand.
I hold cutlery with the knife in my right hand like right-hand people and wear my watch on my left wrist. But I am left-footed when it comes to kicking a ball and juggle the left-handed way.
If your ds is going to learn an instrument, if he does it left-handed his tutor will have to take that into account (I expect most would). But if he learns the guitar it would be worthwhile trying him out with a right-hand and left-handed one before buying him one - see which works best for him. Or a right-handed guitar can be restrung for a leftie.
A ruler is a ruler, it helps you draw a straight line. If you use it to measure lines with, find out which side is easier for him to start measuring from and get him the appropriate ruler if you want to. I always used a standard ruler as measure from the left, it's logical to me, but other lefties find it easier to start measuring from the right. It will just take observation over time to realise which things he does in which way, I guess some right-handed people will do some things left-handed, my dh wears his watch on his right hand!