He died in possession of a sizeable amount of land in his own right i.e he held it freehold, it wasn't land owned by his parents or leased from another freeholder. An IPM inquired into the possessions of tenants in chief so that the Crown could claim any rights due to it and the legal heir established and recorded. The heir would have to pay relief in order to take possession. (All land ultimately belonged to the Crown).
As his son was still a child, the crown would have had wardship rights.https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/inquisitions-post-mortem/#2-what-were-inquisitions-post-mortem
