The reason that c++-mode does not use the /* ... */ syntax is because the variables comment-start and comment-end are set to "// " and "" by default in C++-mode. Changing them to "/* " and " */" doesn't quite achieve the effect you asked for, because then it always uses the /* ... */ syntax instead of "//".
To use /* ... */ only for regions in c++-mode that contain multiple lines, you can do something like the following:
(defun my-with-c-sytle-comments (orig-fun beg end &optional arg)
(if (and (member major-mode '(c-mode c++-mode java-mode))
(not (looking-at-p "[ \t]*$"))))
(let (;; Set up comment style to use "/* ... */" instead of "// ..."
(comment-start "/* ")
(comment-end " */")
;; Use a single pair of "/*" and "*/" for the entire region rather a
;; separate one for each line.
;; Don't insert a "*" at the beginning of each line. This can't be
;; blank because then Emacs will just ignore it and revert to the
;; default behavior of inserting the "*"'s.
;; Remember the first and last lines of the region commented.
(first-line (line-number-at-pos beg))
(last-line (line-number-at-pos end)))
(funcall orig-fun beg end arg)
(when (< first-line last-line)
;; Delete the comment-continue string at the start of each line after
;; the first one.
(forward-line (1- first-line))
(while (< (line-number-at-pos) last-line)
;; Make sure that the line starts with comment-continue plus a
;; space before we delete the prefix.
(when (looking-at-p (regexp-quote (concat comment-continue " ")))
(1+ (length comment-continue)))))))))
(funcall orig-fun beg end arg)))
(advice-add 'comment-region :around 'my-with-c-sytle-comments)