DESPITE HIS devastating illness, Michael J. Fox is joyously expecting his fourth child – – and the good news has strengthened Michael’s belief that he will be cured of his Parkinson’s.

Devoted family man Michael has struggled to keep his energy-sapping Parkinson’s disease from interfering with the time and attention he lavishes on his beautiful children.

“There are both joy and tears in the Fox household now that Michael’s wife Tracy is expecting their fourth child,” disclosed an insider.

“While Michael’s thrilled that another little one is on the way, he’s also scared to death.

“Family means everything in the world to Michael.

“As it stands right now, he is barely able to keep up with his 12-year-old son Sam and his twin 6-year-old daughters Aquinnah and Schuyler.

Michael believes there will be a cure for Parkinson’s by 2010

“Michael could be wheechair-bound by the time the baby is walking.”

The remarkable actor — who’s become a leading spokesman for the war against Parkinson’s – – is placing his bets on the scientists.

“Michael remains ever positive,” confided the insider, “and he insists they are on the verge of findng a cure!”

Sadly, Michael can no longer go out and play one-on-one hockey with his beloved son Sam.

Remembered for his snappy, sparkplug performance on TV and in the “Back to the Future” movies, Michael is now so weak that “there are days when he drags around the house like a feeble old man,” disclosed the insider. But he refuses to give up.

Michael, 40, and his lovely wife — 41 on June 22 — are expecting the blessed event in October and a spokeswoman says the upbeat couple are “extremely excited, extremely happy.”

The plucky star — who was driven from his “Spin City” hit by the degenerative neurological disease — is frequently in pain, slurs his speech, limps and drags his left leg.

“Michael’s always been such a loving, physically affectionate dad. And he absolutely dreads the day he’ll have to settle for watching his kids playing in the yard from a window,” revealed the insider.

A former “Spin City” regular divulged, “Michael loves his children more than anything else in the world. Everything came to a screeching halt whenever his twins showed up on the set and he’d take them into his arms together for big hugs.

“One day, he said his main worry was his Parkinson’s desease would rob him of many of the joys of fatherhood. Michael told me, ‘That would be a cruel blow.’ “