Obviously this is just whining by a butt-hurt narcissist, but if Trump manages to be re-elected I would have little problem with him assigning a special prosecutor to investigate Biden.
If Biden has profited by his son's dubious activities, it would be good to uncover the facts (and I presume an independent prosecutor would do a better job than a congressional committee).
And any allegations that produced indictments would have to go through the judicial process, just as Trump's will.
If Biden is truly innocent of any wrongdoing, I think a good defense and a jury trial is likely - although never guaranteed - to produce a not guilty verdict.