A tenacious, energetic midfield player, Keenan Forson joined West Ham United at the age of 14 following a successful trial.
He began his scholarship with the Academy of Football in the summer of 2018, having broken into West Ham United's U18 squad the previous season.
Forson's U18 Premier League debut took place on 21 October 2017, in a 1-0 home win over Reading, and he tallied five appearances at that level while still a schoolboy.
As a first-year scholar, the teenager was a regular in the U18 squad in 2018/19, appearing 17 times in the league. An U23 debut also arrived in October 2018, in a 2-1 Premier League 2 Division 1 defeat at Everton.
Forson began the 2019/20 season in impressive form, earning himself a call-up to Dmitri Halajko's U23s squad in early September.
The then-17-year-old entered the field in the unfamiliar position of right-back with West Ham United U21s trailing 4-1 at half-time to Newport County in the Leasing.com Trophy.
His impressive performance helped nullify the hosts' wide threat in an eventual 5-4 turnaround, earning him a start in defence in the 1-0 win against Valencia 'B' in the Premier League International Cup a week later.
After impressing during 17 appearances for the U18s during the curtailed 2019/20 season - including opening the scoring against title rivals Chelsea in a 2-1 win -Forson signed his first professional contract in the summer of 2020.
The midfielder initially struggling to establish himself within the U23s squad and only appeared on the bench until November of that year, but when Forson’s opportunity did arrive, however, he grasped it, making such an impression that he went on to record 17 starts in the U23s’ subsequent 19 games, scoring three times in the process and being widely lauded for his role in the U23s sealing survival upon their return to Premier League 2 Division 1.
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.