what was his rank then?
He was an esquire and was knighted later.
Thomas Erpingham
Erpingham joined John of Gaunt's service in 1380 as an esquire, and was knighted by him.
when did thomas erpingham start his military career? 1368
wiki:14157715
Thomas Erpingham [SEP] Military service
Erpingham's career as a soldier led him to serve in France during the Hundred Years' War, in Spain, in Scotland against the Scots, in Prussia and the Holy Land, and spanned nearly 50 years. Beginning with his service in Aquitaine under the Black Prince in 1368 and concluding with his role at the Battle of Agincourt, his military career was linked with the Lancastrian dynasty. Erpingham joined John of Gaunt's service in 1380 as an esquire, and was knighted by him. He followed Gaunt to Spain in pursuit of the throne of Castile. After becoming a retainer of Gaunt's, Erpingham served in the Scottish campaign of 1385, against the Duke of Brittany at the relief of Brest in 1385, he fought again in Spain in 1386.