Just speak to him all the time in your language all the time from now on. Total immersion is the quickest, easiest way, especially with a child so young.
You could start by teaching him some childrens' songs, or just some simple everyday nouns - for example, by writing a shopping list together. Or talk and read aloud with him about something he's already interested in (eg Star Wars, Spiderman, Lego whatever it is he's into, but only use your language).
And leave him alone with your mum sometimes when she visits or when you go to your home country. I don't think you need to do any specific teaching when you're a native speaker yourself.
YouTube is good as an extra, but kids learn languages a lot faster from real live people than from watching things on screens. And it can be a fun, special thing to share together.
I know some kids who moved country and had to pick up a 2nd language at ages 6 & 4. It took them about 6 months to get confident and maybe 10 months to be fluent and now, not yet 2 years later, you would think that they were born here (until you hear their parents, who have not learned so fast...)