No, he was subject to the 1948 BNA under which he MAY be illegitimate due to the POSSIBLE bigamous marriage of his parents and the PROBABILITY that he never legitimated under the requirements of 1948 ACT to gain British citizenship.
If the HI marriage of his parents was not bigamous, then to me he would clearly not be a NBC due to dual citizenship, as I agree with Leo's analysis with the exception of his assumption of a non-bigamous marriage.
The dual citizenship would merely drive the point home, because the foreign citizenship is British, freighted with all the negative connotations to the Founders as it would be. It paints a more compelling picture, in other words.
It’s not the dual citizenship at birth itself that precludes natural-born citizen status, though, it would be the circumstance under which that dual citizenship arose, which was having a father who was not himself a citizen. The jurisdiction at birth would not be complete. This would be true, whether or not literal citizenship was legally conferred by the foreign jurisdiction or whether it was not.