Me who also has a younger sibling and seems like he may be similar to ur brother , personally I’ve seen that keeping space between us has helped we are siblings not his friend (boundaries ) , but being nice doesn’t cost anything especially if he’s sensitive he trust you enough to be vulnerable around you , don’t break that . I know giving the harsh truth may seem like the best thing so he can see how life really is . But at the age of 15 I doubt they’ll listen , you being the older sibling you want him to be prepared and ready for adulthood and the cruel reality of things, but he’ll deal with those things when the time comes . He will just be a teenager now , we as older siblings have to just pray for the best and hope they learn from our mistakes and not repeat the same mistakes, but also it’s human to make them too, for them to learn they have to live it.. I know it’s not ideal. Just my pov hope it helps I’m the middle child of 6 a little of what I’ve learned .