A poorhouse in Scotland is more or less equivalent to a workhouse in England. It seems likely that Neilston Poors Asylum was another name for Neilston Poorhouse.
People who, for whatever reason, could not afford the basics of food, clothing and shelter could apply to their parish for help. Until 1845 this was done by the kirk. From 1845, the task of dealing with the poor was carried out by a parochial board.
When someone applied for relief, they might be given a small sum of money with which to look after themselves, or they might be taken into the poorhouse. In the case of an orphan, it would normally be the latter, though young children were often boarded out with suitable familes for a small weekly fee.
The parochial board for any parish was responsible for providing a poorhouse. Sometimes a group of parochial boards would combine to provide a joint poorhouse to serve all their parishes.
I don't know which transcription you are using that lists him as Frevas (probably Ancestry?). The index at Scotland's People has Trevas, so it is likely that your transcription is wrong. You should look at the original and make up your own mind what you think it says.
He's in the 1851 census (SP index) as Henry Jarvies, 3, pauper, in the household of Margaret Deans, 65, pauper nurse, in Neilston. This specifically means that he was being looked after by the parochial board. If the Neilston parochial board records have survived, they may contain quite detailed information about him, including details of his parentage.
Start at
http://www.rootschat.com/links/01rj4/ - if they don't have the Neilston parochial board records they should know where they are, assuming they have survived.
