When it comes to Facebook, don't even think about using that platform since it's not really trust worthy since "Zucc must Succ" lizzard boi, did lie so badly at the supreme curt about selling user data and other stuff.
If he or you don't want to use your phone numbers (even though Signal only saves information like "account created date&time", phone number used during the account creation), you could also use Keybase, where you can share larger files like videos or perhaps encrypted information but there are two problems..
The messages are synced between the devices unless they're set to self destruct (I just tested that on a account from a friend of mine)
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and you can recover an account if it's installed on other device or stored the recovery phrase somewhere unsafe
(this won't work if you formatted/factory reset your phone or just deleted your user account)
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However, I don't know how safe this IM provider is.
I know a couple of black market dealers from a referral Keybase group since 2018 but yeah.. I'm more familiar with Signal..
Depending on the country.. In my country, I would definitely get in trouble for that but like my friend joked before he died in an accident with his "non-street-legal" Subaru, it's not illegal if you don't get caught..
Signal would definitely be my main app to go for this whole "thing". You would only have to delete the app before "doing it".