A VCard can be imported as a file attachment to any deltachat client, sent to self in pinned chats, then clicked to add a contact. Can you send him a VCard?
Tuta is considering adding Autocrypt support, and is a drop-in replacement for Protonmail.
All but spearphishing spam would probably not pay if you had to encrypt each message.
The introduction mechanism for joining groups makes diverse key exchange methods riskier.