Since entering treatment for substance abuse, Stamos is "focusing on his health," a source close to the actor tells PEOPLE

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Credit: Mario Anzuoni/Reuters

Between his upcoming Fox series and the highly anticipated Full House spin-off, John Stamos is having a particularly busy 2015 – which is what made the news that the he’d checked himself into a residential program for the treatment of substance abuse all the more shocking to his co-workers.

Those who have worked with Stamos, 51, call him “lovely,” “charming” and “always professional,” but after his June 12 DUI arrest, a pal told PEOPLE that the actor’s drinking had been a “problem” in the past.

A source close to Stamos adds that the death of his mother, Loretta, last September “did a number on him. It s been a hard time.”

But after entering treatment in late June, Stamos is “focusing on his health,” says the source.

“He has a big year ahead of him,” continues the source. “And he knows he needs to take care of himself.”

RELATED VIDEO: John Stamos Spotted for First Time Since DUI Arrest

For more on Stamos’ decision to seek treatment, pick up this week’s issue of PEOPLE, on newsstands Friday