Taking Rosie's reply a stage further, he was posted to 5 Training Battalion, Royal Engineers (TBRE) based in Chester on 28 Oct 1941, and then to 2 TBRE 0n 17 December 1941. On 3 Jan 1942 he was transferred to the W (Territorial) Reserve and remained there until recalled to regular service on 12 Aug 1943, rejoining 2 TBRE.
This 18 months in the Reserve having not completed his initial training suggests that he was either seriously ill or injured, necessitating his transfer. I would guess an injury was the cause. A major illness requiring that amount of absence would likely have made him unfit for further service.
As for his Merchant Navy service there is a CR1 card for someone of that name (ie George Frederick William Kitto, born 1910 Fowey) in BT349 at the National Archives, with a discharge number of R18657 dated 1921. However he would have only been aged 11 at the time, and was not supposed to have left school until he was 14. This seems odd. Possibly he was at a school for training seamen, of which there were several around the country. More on how to research Merchant Navy sailors here:
https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/merchant-seamen-serving-since-1918/