The thing with delusions and delusional thinking as opposed to flat out hallucinations is this: hallucinations can often be pretty clearly identified as hallucinations:

Do you hear/see/smell/feel that? No? Ok, so that does not exist in our shared reality and is a product of one person's brain [or ingested chemicals or what not].

Delusions are a bit different because there can be a grey area and often they require a comparison to the norm.

One of the responses to the OPs question was that this is not delusional thinking per se.

My response is that the response to have a heightened fear, anxiety or exacerbated response to a police or any general authoritative presence would not be abnormal in this given context.

For it to be the focus of obsessive thinking and to interfere with ADLs or life in general- particularly to the point where it prevents functioning in any major categories- social, hygeine, general physical health, etc? THAT is where it is a problem.

I would also suggest examinging the situation in this way, OP:
You are aware of these stressful and frightening situations and these factors that are leading your son to be fearful to the extent that it is interfering with his life.

Are you, on his behalf, so fearful that he may be randomly approached by, accosted by, possibly arrested/abducted by the police or another authority that you think it reasonable for his life to be disrupted to this extent?

Or is YOUR life disrupted to a great extent based on these fears for him?

The reason I would ask you to maybe look at the situation from that angle is that gives a good comparison to see what may or may not be reasonable or appropriate response.

If you don't share the same life-disruption? I would say that, actual clinical delusion or not, there are major problems. You can' diagnose them, but you see that he is falling down a hole. He is still sixteen, he is still a minor, so you have the power to help him whether he wants it or not. Before he becomes a legal adult, I really suggest you entertain the idea of addressing this issue and pushing addressing the clear alienation these fears are causing- WHICH ARE VERY REAL TO HIM.

The longer they go on, the more problems they cause and the harder they will be to address. The second he turns 18, you legally can't do anything to change that unless he wants you to, basically.

I know that wasn't your question, but I think it's important to address that. He's probably a whole lot of scared about a lot of things that he may not even share.

You probably know that. But he's almost 18.