A tenacious, energetic midfield player, Keenan Forson joined West Ham United at the age of 14 following a successful trial.

Forson's strong form continued into the 2021/22 season as he took a central role in helping the U23s go to the top of the Premier League 2 Division 1 table with series of influential performances. He also netted in a 2-1 EFL Trophy group-stage win at Ipswich Town in September 2021. His reward for those displays was a first-team debut as a late substitute in the 1-0 UEFA Europa League Group H defeat by GNK Dinamo Zagreb at London Stadium on 9 December 2021.

Appiah-Forson would end the season with more appearances for the U23s than any other player, featuring 34 times in all competitions for Mark Robson's side. The midfielder's energy and tenacity saw him rewarded with a new contract in early 2022, extending his stay with the side until 2023.

Appiah-Forson's significance to the development team continued in 2022/23, with the midfielder again making the most appearances of any player for the side, with 32. He would add another first-team appearance too, coming on as a substitute in the UEFA Europa Conference League Group B win away to FCSB in November 2022.