The former Henry Cort Community College will link up with the young Lions next month.
It's reward for Steward, who has risen through the ranks at PO4.
The 16-year-old started his two-year scholarship at Fratton Park this summer.
Steward also helped Henry Cort Community College claim the delayed 2019-20 Year 10 Hampshire FA Cup in May.
The Fareham-based school beat Portsmouth’s Admiral Lord Nelson 2-1 at Cams Alders.
Steward pulled off a string of saves to help deliver his side the silverware.
