When you say you are frightened OP, do you mean you are scared he'll be violent? Or do you mean you are just scared he'll shout at you again, or snap?
It depends if you want to continue contact. It's hard to let go but what do you get out of continuing contact with him? Has he always been like this? How old is he?
The fact he's messaged you to tell you he's bought a new motorbike suggests he wants contact and interaction. I think not replying to the message will piss him off, and understandably so. It's a bit rude. If you want to continue a relationship and have contact with him, then you reply in the way you feel is right, but make sure you are calm and not accidentally being inflammatory or rude. My DM has a big habit of being outrageously rude and insulting to people without realising she's doing it. A simple "that's great, really pleased for you" can't be misconstrued.
And if at any time he gets shouty or rude, you calmly disengage and end the conversation. Every time. He needs to get the message that he can't talk to you like that. You either message or you say, "I'm finishing this conversation now if you're going to shout at me. Goodbye" and hang up. Said calmly - very important you don't get sucked into shouting back. And you really mean it, you hang up - no waiting around for him to shout or dispute your interpretation. End the call. Finish the messaging and leave him on unread.
He won't like it and may decide to drop/reduce contact if you're calling him out on his abusive behaviour. But it's not fair for you to be shouted at if you genuinely have done nothing to provoke it. I'm sorry this is happening, my DM has a difficult relationship with my brother, and I know how hard she finds it.