Mark: Since you must know the names of his parents, try this:

On the IGI type his parents names in the boxes on the right side; you only have to have his father's full name and his mother's first name - and - do NOT put his name in any boxes. Then click on births/christenings and British Isles - Isle of Man. This will bring up any other children these parents had.

You might then have to sort out the obvious wrong names/dates if there were several other parents with the same names. There is another better way to do this with Batch Nos., but I'm not familiar with the procedure. Maybe someone else can help?

Good luck.