Simplify the following text for a Grade 9 native English-speaking person.
Original: From 1988 to 1995 , he was principal conductor of the London Symphony Orchestra , and since 1995 has held the title of principal guest conductor with the LSO .
Simplified: From 1988 to 1995 , he was principal conductor of the London Symphony Orchestra , and since 1995 he has been their main guest conductor .