Leaseholder wants to split his flat into four smaller ones

I am the freeholder of a mixed use property containing 5 flats and two shops. Two of the flats, and one of the shops have 999 year leases dating back to long before we bought the property. (The rest are mine). One of the long lease flats has recently changed hands and the new owner wants to develop it. It is a large flat, and he wants to convert it into four one bedroom apartments.

His lease provides for single family occupancy only and contains the usual clauses about requiring the freeholders consent to alterations.

I don't want him to do this - I can foresee numerous problems with noise, security and parking, but his lease states that my consent is not to be "unreasonably witheld"

So, would I be being "unreasonable" in this case? He says I am, and the last thing I want is a huge legal bill telling me the same thing :(