Yes, you can apply for a declaration that the death happened at a date earlier than the order and, depending on the available evidence, you may be obliged to do so.
Where the court is satisfied that the missing person has died, the court will make a finding as to when that was. If they died at some point during a known period, the court will declare that they died at the end of that period.
Where the missing person has not been known to be alive for more than seven years, the court will declare that they died at the end of that period.
In a case I did, the missing person left home apparently in the clothes she stood up in and was never seen or heard from again. The conjecture was that she had went for a walk in the woods nearby and met with some misfortune. The court declared that she had died at the end of one week from the date on which she went missing.
In your case, if the son disappeared in 1987, it sounds as though the court is likely to declare that he died in 1994. The declaration rules are expressed in mandatory terms, so that it doesn't seem to be possible to get a declaration for a different date, even if that would be more desirable for tax or succession reasons.