Does the HTTP Protocol support multiple content types in response headers?

No. As pointed out in the accepted answer, the header value itself does not allow for multiple values, and it is also not allowed to send multiple Content-Type headers:

A sender MUST NOT generate multiple header fields with the same field name in a message unless either the entire field value for that header field is defined as a comma-separated list [i.e., #(values)] or the header field is a well-known exception (as noted below).

https://www.rfc-editor.org/rfc/rfc7230#section-3.2.2

None of the "escape clauses" holds, because media-type does not allow a comma-separated list of values, and Content-Type evidently is not a well-known exception, either.

You need to look at the definition of the header field:

http://greenbytes.de/tech/webdav/rfc2616.html#rfc.section.14.17

Content-Type = "Content-Type" ":" media-type

so it takes a single media-type, which is defined by the grammar you quoted above.

So the answer is: a single type/subtype, followed by optional parameters.