I particular he believes that English grammar was highly influenced by Welsh grammar in its (English's) early days in England
Hard to say one way or another without seeing the evidence. Conjecture is fine, as are SWAGs, and I do enjoy McWhorter's linguistics, if not his politics. Off the top of my cold-addled head, Celtic languages are VSO and Germanic ones tend to be SOV changing towards SVO.
His argument on the Welsh influence is based on the use of what he calls the meaningless "do", in particular in questions and the negative. E.g., "Do you come here often?" as opposed to the normal IE "Come you here often?" or "I don't drink milk" versus "I drink not milk."
and that Proto-Germanic was influenced by a Semitic language in its early days, citing the fact that about one-third of the words in the Germanic languages are not from any known PIE roots and that the ablaut series for strong verb tenses is seen in no other branch of Indo-European.
This same, well-known situation, has led others (e.g., Theo Vennemann) to suggest that Germanic developed from a creole mid-way between (Pan-)European and Proto-Indo-European. There was a philologist in the 19th century who studied the Semitic substrate in Indo-European. It makes sense because a lot of technology and products came out of the Middle and Near East.
OTOH, ablaut is a huge part of Indo-European phonology and morphology. It also exists in other languages (e.g., I have a book on ablaut in Moroccan Arabic). I have not read the book you link to -- yet.
Is the ablaut used to mark tense in any non-Germanic languages?
