can he sell his interest in the property in Stonehouse, and then use the money to buy a property in Glassford or elsewhere if he wants.
Yes, once any liferenters have died, he can do what he likes with it.
There should be sasines recording these changes of ownership. The registers of sasines record changes of ownership of heritable property, which is basically land and buildings. Normally all you need to do is look at the Abridgments, which give you brief details of the property being transferred, who is transferring it to whom, and how (money, inheritance etc). Often, but not always, they tell you the relationships between the people involved.
They are in the National Records of Scotland, but they aren't available online, so you'd need to find someone willing to look at the relevant ones for you.
The following is an example from my own family of a useful sasine with lots of information about family members:
Lanark No 5139 1806-09-12 GR 764/200 Jean Waddell, sister of William Waddell of Easter Moffat, and William Waddell, son of George Waddell, of Ballochnie, their nephew, seised in liferent and fee respectively Sept 9 1806 in the 40s land of Easter Moffat, par Bartramshotts, 1 merk land of Easter Calderhead and 20s land of Paperthills, par Shotts, and teinds, under burden of a liferent annuity of £20 to Christian Waddell, spouse of James Muir of Gilgarth, £2000 to Margaret Waddell, daughter of Patrick Waddell of Bogo and £100 to Agnes Gairdner daughter of John Gairdner of Broompark on disp and settlement by the said William Waddell, May 6 1803. William Sr, George, Patrick, Jean, Agnes and Christian Waddell were siblings. Neither William nor Jean married. Christian married James Muir of Gilgarth and Agnes married John Gardner of Newmains. I have yet to research the link between Newmains and Broompark. William Sr drew up a disposition and settlement dated 6 May 1803, and died in 1806, so this is the record of the change of ownership of the property. Jean got the liferent, and when she died it was to go to their nephew William Jr, son of their brother George.
Does that help?