This is so interesting. Are you sure he was deceased in 1844 as my Peter was born in 1837. Didnt you say his son was born in 1862?
A man named Peter Miller Watson was the co-owner of the Plantation called Zeeburg with Alexander McLaren, so that would make a definite fit. This Peter Watson was a slaver, plantation owner, and sugar exporter.
My Great Great Great Grandfather Alexander Mclaren was born in Killin, Scotland, but went as a teenager to Demerara to make his fortune. He started out working on plantations for other but eventually owned two. He was married twice, first to Anne Lockett and had one child. Ann died in childbirth and we dont know anything about her. His second wife Fatima was said to be a free black woman, (at least at the time they married) and may have come from Jamaica. Therefor the children were half black. Alexander moved back to Scotland around age 50 and purchased two estates. John Patrick and Fatima stayed in Demerara to look after the estates there and they shipped sugar to Scotland. Doanld stayed in Scotland and was a ship captain. Mary married a doctor called John Coppleston Baker and they lived in Liverpool. Isabella married a Mr Smith and eventually their family moved to Australia. Annie married a lawyer named Alexander Philip and they moved to Canada. Annie is my ancestor. I am going to email you a few photos, one of Alexander, one of Fatima, one of Annie and one of Mary. Annie and Mary would be sisters to Peter. Did your Peter look like he may be part black? The one of Alexander Baker would be a first cousin to your John. (if this is the right family)