I think it’s one of those things that just takes time. I had it with mine and they’ve only just started getting along better and playing together. When talking to the youngest, praise and say nice things about your eldest. Also ‘translate’ what your youngest is saying like ‘you really love your brother’ ‘you think your brother is the best’ ‘you really like playing with your brother’. Also, I don’t know how old your youngest is but get your oldest to hold him every so often and get them to hug too.
Most importantly, keep going you’re doing all you can. It will get better!
Going through something very similar but he will scream in his face until he cry’s (baby is 6 months) and then laughs. Snatches toys off him all the time and is very rough with him. Was told it sounds like sibling jealousy but not got a clue what to do as he gets attention etc!