My thoughts

Honestly, it seems not a big deal to me. However, if you are really concerned especially since it happens again and there is a clear pattern you believe, you probably want to focus on the following which is under your control:

1. It is good for you to bring this to your boss's attention but you probably shouldn't keep bringing this same concern to your boss since he seems not concerned and your boss most likely has more important work to do. It is nice if he wants to spend more time on helping. If not, it is normal as well. After all, anyone wouldn't want to spend much time on hear and say. I don't think your boss takes anyone's side. Frankly, I would do exactly same thing in this situation.

2. You probably also want to start to answer work related questions in emails instead of just talking verbally; It maybe not most convenient and friendly but it is a good way to document what really happened and you helped. Or, maybe your coworker couldn't remember everything you told him so he had to reach out for a formal written document? Or, he just wants to ask formal documents in case something missing? It could be his personality as some people are more detailed than others.

3. You could start to create some process documents so it helps other people understand your work given you are senior I guess. Even they might not need to do your work but only need your results to do their part of work - it is good to understand the results before using it, etc. And creating some document is your credit and contribution on the table loudly. It also prevents the coworker from possible complaining about you due to his own inability to leaning things.

4. If you are concerned your job security, keeping things to yourself is not helpful. Instead, it might motivate your manager to find a replacement quicker.

Just my two cents and good luck!