There were an Ahab Sayle & Ann Crellin having children bapt in Kk German around that time, but some children may have been missed (which often happened). They had an earlier Thomas, but according to the IGI he died young. You need to check the actual register for clues - can be seen at the Manx Museum library or on film at a LDS Family History Centre. St Peter's Church, German: bapts & burials, 1783-1826; baptisms, 1827-1856. Film no.106723. Marriages 1757 - 1849 film no. 0106724.
There was a Thomas Sayle marriage in German in 1829 [IGI].

I have just checked Brian Lawson's website, and there is an 1811 will transcript for mother Ann [Crellin] there: http://www3.telus.net/lawson/twill/1811_029.html
- Which names her husband and includes a son Thomas, so it looks as though my guess is correct.
You need to check the register to see whether this Thomas is the one bapt 1790 (and reduced his age for his second marriage) or whether he really died in 1791 as given by an unknown researcher, and your Thomas was born later.

Sue