My first thought is: “Did the draftsperson erroneously leave out the gift over?”
I suggest the will instructions are obtained and reviewed.
In the absence of guidance from the will file, it is more likely that the remainder will pass to those entitled upon the intestacy of the deceased. If the son is the only possible beneficiary under intestacy, then the interests will merge into an absolute interest for the son.
However, if the son is only one of a number of beneficiaries who would be entitled under intestacy, then his proportionate share of the remainder will vest in him and his life interest in that proportion will be an absolute interest (and the rest of the trust fund will be subject to his life interest).
It might be the deceased intended his son to have the entire estate, but thought he could protect the capital by purportedly only giving the son the right to access the income. Whilst I have seen examples of this in a home-made will, it would be unlikely to appear in a professionally drawn will.