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.
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 alos 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.
