The simple answer is that peers generally don't do that, because real
titles are overwhelmingly preferred to invented (or extinct) ones.
There would indeed be nothing stopping him doing that (although I
would think it more likely that if His Grace were desperate for a
higher title for his heir he would style him Marquess Seymour or Earl
Seymour, merely raising his barony to a higher degree (or using an
invented surname title, depending on how you look at it) rather than
reviving an extinct title - and of course the Earldom of St Maur
[which, incidentally, was simply "Earl St Maur" rather than "Earl of
St Maur"] was created at a time when the Dukes of Somerset were
calling themselves "St Maur" rather than "Seymour"), but then there
would also be nothing stopping the Duke of Norfolk styling his heir
Marquess of Arundel (or Marquess Howard), or the Duke of Manchester
styling his heir "Marquess of Mandeville" and his second heir "Earl of
Kimbolton", and so on and so forth, but they generally just don't. Not
a very satisfactory answer, unfortunately, but then as I'm sure you
know these things all too often boil down to "that's just the way it
is"!