I'd look at his life and see where you can support him.
Do you know what he's doing online? It would've a good idea to investigate if there is any online bullying, review his devices.
Make an appointment with the Deputy Head for Pastoral - don't say what it is just send an email saying you have very serious concerns and need an urgent appointment. Then follow up in writing after the meeting with your concerns and the fact you want school to check what is going on.
Does he want to move school? Even when being bullied it is ommon for children not to want to move. This could be stressful.
What is his life like outside school? You say he wants to go online - what other activities could you put in place and what extra stuff can you do with him? Could he be lonely? I'd be looking at playing board games, card games, watching a long series together, cooking, gardening, exercising together.
Do you talk to him about the emotions rather than the causes? He is at a hormonal age which makes feelings stronger, but that doesn't mean they're not real. It can be very hard to tell. Get him a teen book about emotions and use the right words yourself. Don't pressure him for info but say things like 'If you're sad, I am always here to be with you.' If writing has worked before, do it again.
TLDR: investigate both at home and school, add in loads of positive attention, provide emotional support.