C.S.
Even though most child abduction cases are caused by close family or a relationship turned bad, there is no such thing as being too careful. Since your son is 22 months start talking to him about being in the company of a trustworthy caretaker like you, or your husband.
When strangers ask him personal questions like where he lives and other questions you think are inappropriate change the topic or distract him before he gives the answer. Believe me - it is not worth being polite to a stanger if you see his safety is being compromised. Go by instinct - in most cases it is right.
If it worries you so much, dont have a routine for outings. If today you went to the park at 10, take him there tomorrow at 9, before someone abducts a child they usually study their pattern.
I hope I was of some help.