It looks like the correct IPM Darlene - Thomas Grene, knight starts at section 41 on this page and ends at section 46.
The date of the writ is 1 September, 15 Richard II, which converts to 1391. (Regnal year calculator here)
All writs confirm that Thomas Grene aged 22 is his son and heir.
The writ from Wavynden, Buckinghamshire is section 45.
Concerns the manor of Wolston and Wavyndon (WW)
Historically a Henry Grene has a son Henry Grene. WW given from Henry the father to Henry the son, with reversion back to Henry father and his heirs when Henry father dies.
Reversion happened (on the death of Henry the father) and WW went to Thomas (the subject of this IPM) son and heir of Henry the father, implying that Thomas is older than his brother Henry. At time of the IPM, Henry the son (brother to Thomas the heir, subject of this IPM) holds WW together with Maud (Henry the son's wife).
Henry and Maud also hold the manor of Emberton from Thomas in the same fashion.
The research notes say that Thomas Greene-404 is the son of Henry and Maud, but it is clear that in 1391 that Henry and his wife Maud were both alive.
I also note two sets of duplicates in Thomas' children - Joan (same dates and married name) and Walter (same death dates, but differing birth dates)
Edit: corrected spelling of Wavynden