Search online for "Halloween events" in the national newspapers--the Guardian, Telegraph, etc. and absolutely get the magazine Time Out London and check its family events listings nearer to Halloween. This early in the year, you won't get the 2017 events listings, but you can see what kinds of festivals and events have been done in the past. Lots of events are at historic properties, museums, amusement parks, local parks, etc., doing Halloween themes.
In London there will be lots of events but I would set his expectations that if you're in a hotel he may not get to trick or treat like at home where can go door to door to people you know.
Halloween has gotten a bigger foothold in the whole U.K. than when my husband, who is English, was a kid and teen there. Our family is in a more rural area and there are things in the towns and villages like haunted houses (usually for charity), costume contests, events at local historic houses etc., but little trick or treating. That's just our area, though.
In London you can also check with the museums which sometimes have kid and parent sleepovers inside the museum for special events--these are expensive and you'd need to reserve from the US long before the trip, but our niece used to love museum sleepovers....The British Museum did them and I figure others might too.
If you're there through Nov. 5, be sure take your child to a local Guy Fawkes night bonfire and fireworks that night. It's much more traditionally English than are Halloween celebrations, which can be a bit Americanized....the good thing is, you can fit in both!
Smithsonian magazine has an article online (from 2014) about how increased focus on Halloween has affected the historic Guy Fawkes celebrations. You might find it interesting to look up.
Have fun!
