"I think that he needs help and that attention needs to be put to his health," drummer Joey Kramer tells PEOPLE.
And Kramer says he isn't referring to the broken shoulder Tyler sustained when he fell off the stage during a concert in August.
Asked if he thinks there is something else going on with Tyler, he says, "I do."
Recent reports in the Boston Herald and online suggest Tyler, who battled drug addiction in the '70s and '80s and checked into a rehab center last year to detox from painkillers, may be partying again.
Though Kramer would not say whether he has seen Tyler use drugs recently, he says he is concerned because the singer has "isolated himself."
Says Kramer, "He's got some bad influences in his life right now and he's making poor choices."
Aerosmith guitarist Brad Whitford tells PEOPLE, "His behavior is spinning out of control."
Tyler's bandmates say they haven't spoken to him in weeks, but last Tuesday, he unexpectedly showed up at a concert given by Joe Perry. Tyler performed one song with Perry, then took off without saying goodbye.
"I don't know where he is and what he's doing," says Whitford, who admits he, too, is "concerned about [Tyler's] well-being and his health."
Cancellations GaloreThough the band had been touring earlier this year, Whitford says that Tyler's behavior has been "erratic" and "unpredictable." He notes, "It wasn't an overnight change. It's been progressively getting out of hand."
The result? "Loads of cancellations, canceled tours, and canceled dates," says Whitford.
For his part, the singer has said he wants to focus on his own brand and possibly record a solo album; he is also working on a memoir. In an August interview with Rolling Stone, he maintained he was not on drugs and said he was "as sober as can be" when he fell off the stage.
On Monday night, he was spotted at The Pump Energy Food in New York City, where he ordered a fresh-squeezed juice made of carrots, beets and ginger. The following night, he attended a John Mayer concert with girlfriend Erin Brady. "Steven was having fun, getting into John's music, and loving the attention from his fans," says a source.
A rep for Tyler says, "He's concentrating on his book right now while his shoulder heals."
Meanwhile, Kramer, Whitford and bandmates Joe Perry and Tom Hamilton are considering looking for a new lead singer.
"It's hard for me to imagine getting on stage without Steven," says Kramer, who hopes that Tyler will rejoin them in the future. "He needs to take care of his health first and get help in order for there to be a future for an Aerosmith that includes him."