The 18-year-old is highly rated at Upton Park.
“Frank is an exciting, young player who I am delighted to welcome to the club,” West Brom boss Roberto Di Matteo told the club’s official website.
“He can play in any of the three forward positions – down the middle, wide left or wide right – and we hope the next month is a fruitful one for the club and Frank.
“His main attributes are that he’s powerful, quick and good on the ball but we mustn’t forget he is still very young.”
