The versatile forward, born in Watford, joined the club for the second time in December 2020.
Grieves is the great-great-grandson of former Watford goalkeeper Reginald 'Skilly' Williams.
He made an appearance on the bench for the first team in the 2021/22 season, at home against Everton.
In the following 2022/23 campaign, he featured regularly for the Under-21s and made his league debut against Blackpool, whilst making another appearance against Stoke City on the final day of the season.
