Isaiah, aged 17, was last seen on Tuesday 20 July.
He is described as being around 5ft 4ins tall and of slim build. He has short black hair and brown eyes.
He is believed to be carrying a dark coloured rucksack with him.
Isaiah has links to the Stratford, Newham, and Lewisham areas of London, as well as links to Ipswich and Braintree.
Officers are growing increasingly concerned for his welfare.
If you have seen Isaiah since he was reported missing or have information about where he has been, you can report it by:
Calling 999 immediately if you believe you are with him now or have seen him in the last few moments.
Filling out our online reporting form at herts.police.uk/report.
Speaking to an operator in our Force Communications Room via online web chat at herts.police.uk/contact.
Calling the non-emergency telephone number 101 if you have information about where he has been.