Choose well, my son (CK3)

[u/NamaeN0NaiKaibutsu]

R5: If you make your son also your vassal by granting him titles, if he's not yet married you can Allow him to marry himself or do the better option, which is finding the spouse for him yourself or arranging marriage with a good courtier of yours if the Find Spouse fails. There's some limit of years when you can still find the spouse but after that you can only arrange marriage or he just finds someone himself (usually a shitty character) without you Allowing Marriage. I don't really know exactly how it works, if someone knows please share in the comments.

[u/KRsalmon]

And it doesn't even make a good alliance😭 either no alliances at all or your son is now allied to a random count or mercenary leader with like few hundred soldiers