My son is in a partnership rather than married to his beloved. Is it possible to reflect this relationship in the Vitals section- or elsewhere?
Yes it is possible for you to indicate a relationship other than marriage. Once your couple are added to FamilySearch, you can just use the edit icon (a small box with a pencil) to the right of the couple and it will bring up an option to add a relationship event. This can be used to indicate that they lived together among other things.
This article explains in more detail how that can be done:
How do I add or change marriage information in Family Tree?
In case you have children in families that also have relationships other than biological, the following article explains how to select the correct relationships for them also:
How do I specify biological, step, adopted and foster relationships in Family Tree?
I hope this helps you put the correct information in for your family relationships.
Further to what has already been proffered ...
ANY Couple has a "Relationship Event" ... not necessarily a "Marriage".
Currently, in 'Family Tree" of 'FamilySearch' there are Five (x5) designated [well, Six (x6) really] "Relationship Events" that can be recorded for a Couple, they being:
Were, the sixth is to simply just to leave the "Relationship Event" as "Blank" (ie. Nothing).
You can take your pick:
That said ...
Whichever one you choose, make sure you add "Notes" to indicate the actual situation.
Remember, the "Notes" are NOT for you; but, for later Users/Patrons.
IF, your Son and his Partner are still, currently, "Living"; THEN, no one but you will 'see' them.
[ Hence, the bit about ... for later Users/Patrons ... ]
IF, your Son and his Partner have passed away (ie. currently, "Deceased"); THEN, that is another matter; as, ALL Other Users/Patrons CAN 'see' them.
Best to have "Notes"; so that, NO User/Patron, either, current; and/or, future, adds a "Couple Event" that never existed (- which they sometimes do); as, they did not know the actual situation.
Just my thoughts.
I hope this helps.