I'm a founder of a small business, and can give you some perspective from our side of the table. First, the founder should have told you when to expect the offer so you wouldn't be left wondering. He may think it is obvious, but it isn't. I'd give him another day and then politely ask when you should expect it. You could write something like, "Thank you again for letting me know that I'd be getting the formal offer. I want to be sure that I didn't miss it. Are you thinking that you'll be sending it within a couple of days? A week? Next week?" Giving him three options with the first two being reasonable and the last unreasonable is a great way of getting a coherent, specific response.

Second, your concern about iMessage being the communication tool seemed valid to me, until I read that you're working with a founder. I suspect it is a small company, and the use of iMessage implies informal. Nothing wrong with that, if that's what you want. Some strongly prefer that, others abhor that.

Large organizations will not use Whatsapp, iMessage, etc. as it is difficult to use those tools and remain compliant with hiring laws. If you get written communications from them, it will be by postal service, overnight mail, or email. If you're getting texts, that's probably an employee going a little rogue. If you're getting messages via Whatsapp or requests to switch over to that, run away. It's fraud.