There are 22 stones in Dalserf churchyard with the name Thomson. The MIs in the booklet I have were transcribed in 1983, so many will, by now, be unreadable. On some, Thomson may just be the name of an in-law, on others there are whole families of Thomsons.
Perhaps your Alexander is in a family plot but isn't mentioned on the stone. (If a stone was there in the first place and if it has survived).
Do you have his parents or grandparents names?