Well, to me xmpp is the way to go, but I guess it's not a matter of opinion, but rather understanding the motivation.
Why is your dad looking for telegram? What caused your dad to look into it? Perhaps digging into those, one can make a case with alternative more private options. For sure one can always argue in general the already mentioned alternatives, and the ones to come are better privacy wide, but it boils down to why is him looking to use telegram. If it's about having friends or co-workers, or a high school set of friends there, I would think there's no way to change his mind, :( But more technology arguments like stickers, better voice/video conferences, whatever, then that's more something that can be argued I would think, the same way if he's just looking for something more private of course.
They both are bad in privacy in one way or the other. WhatsApp is collecting vast trove of data about you, though it can't read the chat itself. Telegram doesn't have end-to-end encryption enabled by default, means anyone have access to the server can read your chat history, though you're last subject to data collection.
If you're doing illicit activity though, WhatsApp is better than Telegram because the chat contents are the evidence those law enforcements are going after, not the connection. They can't arrest you because you make friends with a criminal, but they absolutely can because you have a criminal action recorded in chat history.
But it's Telegram. Avoiding laws, their registered office tells us enough. No encryption (only in a few cases after one's own initiative). I don't see a better option.
I wish I could quit Telegram myself, but with how much it is used for my university and recreation - it is next to impossible right now. So instead of that, I at least tried to minimize harm: treat it as a fully public space and behave as if every my conversation there is watched right now (so that the chance of saying anything remotely dangerous is minimized), use a FOSS client instead of the official one, and try to get as many individuals as possible to XMPP, Simplex or at least Signal.
The thing is...you can't ! This is more or less a personal "enlightenment" ! You can forward your arguments, say what you know and why one is better than the other, but it always comes to personal experience ! If you're not ready to change, you're not ready, even with the best argument in the world, people tend to always change when they personally experience something.
Don't push yourself or others, this will only cause frustration and anger. Do what you can but don't expect others to change/think the same way you are...
I know this is very vague and doesn't solve your actual issue, but that's the best you can do :)
Telegram and Facebook share the fact that they've got lots of content and communities built around them.
I use telegram for my home automation stuff because it's free, the web hooks are super quick, and it supports image upload
But all my family chatter happens on signal.
The best way you can convince somebody not to put their entire life on telegram or Facebook is to tell them that advertisers are reading their conversations and giving them ads based on what they talk about people feel that spooky as shit
I've used Telegram for years and never seen an ad. Their Privacy Policy says ads aren't based on messages
Unlike other services, we don't use your data for ad targeting or other commercial purposes. Telegram only stores the information it needs to function as a secure and feature-rich cloud service.
Telegram offers a tool for advertisers to promote their messages in public one-to-many channels, but these sponsored messages are based solely on the topic of the public channels in which they are shown. No user data is mined or analyzed to display ads or sponsored messages.
It's about the fact that you HAVE TO trust them to not read your plaintext messages, not see your private photos, whereas Signal is open source (ok both server and client side) and end to end encrypted so you don't really have to trust anyone.
No worries! I have reasons for not installing Telegram myself, but I'm curious what your reasons are? What arguments have you made to him so far and what were his responses?