"He could be the brightest, happiest person you've ever met," a source says of better days
Credit: Jeffrey Mayer/WireImage

brightcove.createExperiences(); Richard Simmons has always been the life of the party.

Known for his enthusiastic 1980s workout videos, hilarious late-night talk show visits and outlandish outfits, the fitness guru could still command the attention of sold-out audiences well into his 60s.

“He seemed so high on life,” a source tells PEOPLE of Simmons’ demeanor while teaching classes at his Slimmons Studios in Beverly Hills, California, as recently as 2014.

Dressed in his signature sequins and short-shorts, Simmons – who has revealed he was hospitalized for dehydration on June 3 – would entertain the crowd as he led them in workout routines designed to get their bodies moving and their faces smiling.

“He could be the brightest, happiest person you’ve ever met,” says the source, who attended the classes regularly.

But in the months leading up to Simmons’ abrupt and unannounced retreat from the spotlight a few years ago – which the 67-year-old has attributed to a knee injury – the source admits “there were days where it seemed like there was no life in him at all.”

Though one week he would be jumping around and tickling his students as he led them in a routine, “the next time you’d see him he’d have no energy and could barely force a smile,” adds the source.

The fitness instructor’s emotional extremes were even more evident in the informal group therapy/weight-loss motivation sessions he’d hold before the workout classes began.

“He was often so motivational,” says the source. “But other times, he’d break down and cry with a student. But it didn’t seem out of empathy. When he cried, you could tell it was coming from a sadness within him.”

Simmons’ friend June Park confirms to PEOPLE that the star’s recent behavior is causing concern among those closest to him.

“We are all so worried about him,” says Park, who owns the Los Angeles-based store Wigs Today, which Simmons used to frequent. “People stop by his house, but nobody gets to see him. Nobody knows anything about what’s going on.”

Despite Simmons’ conscious step away from the spotlight, he was forced back into the public eye due to a recent hospitalization for dehydration followed by false reports that he has been transitioning from male to female.

Simmons’ rep Tom Estey denied the reports, telling PEOPLE: “He has decided to live quietly the last few years. He had knee surgery a few years ago and has a bum knee. He is still on Twitter and Facebook and works behind the scenes continuing to inspire and motivate people to lose weight. When he decides to come back, he will come back.”

Added Estey, “He’s a human being. He was in the public eye for over three decades, but he deserves his privacy. He should be able to have a private life. He just needed a break. He is still taking care of the people who need his help.”

With reporting by JOHNNY DODD