Go and take him out for the day. Give him a hug. Let him talk. Get a steer from him about what he needs from you right now. Offer lots of reassurance that even though his mum is gone, his link to you is as solid as a rock, and for life (or as long as he needs/wants it).
My friend runs a walking therapy counselling service and apparently men /boys often like that approach as they can walk side by side talking, which encourages them to share more than actual face to face over a desk/table.
Also worth contacting one of the various charities that provide support specifically for kids who have been bereaved.
Make sure you call him at least once a week (same time, same day) so that he has a predictable time and space he can head for if and when he needs to talk It will give him some much needed stability.
Be clear to him about what your relationship is with his dad. He will need to trust you if he wants to disclose anything re difficulties on that front going forward.
Get him on WhatsApp and send gentle messages and things if no consequence that he might like (stupid stories, pictures of daft dogs and cats etc). It shows him that he is in your mind, without you having to say it.
Make sure your put key dates in your diary (his birthday, mums birthday, her death day) to make sure you contact him on those days to show you remember and care.
So sad. That poor child. Your support will be invaluable to him. 