We started with broccoli & she loved it, still does, but there were loads of things that took her ages to get used to. Avocado she still doesn't like.
Banana we had quite a few goes at & now she will eat almost a whole one at one go sometimes.

Sometimes it's not the taste but the texture, too.

Just keep offering... I think I read somewhere that it can take 100 tastes sometimes for them to accept a new taste (although not always). I would offer a couple of different things in one go if he's not interested just try again later.
It's not hugely important that he eats great amounts of solids right now, so don't stress about it, just let him go at his own pace and he will get there