Ben can be seen in footage being soaked by a man dressed as a chicken last week, before he nominated his mum for the same fate.
Another video, taken yesterday, shows him sitting next to his mother Diane Dernie in the sunshine, before throwing a bucket of freezing water over her.
Ms Dernie, 56, said: “It was really cold but it was great fun. Ben had done it the week before and I was one of the people he nominated so I took the challenge when it was nice and sunny outside. He had to do the water because his hand is too shaky to hold the camera.”
Both mother and son pledged their donation to the Motor Neurone Association.
Lance Bombardier Parkinson, 30, has become a symbol of the bravery of British war heroes as he battled back after losing both legs and breaking his back, hips and ribs in a landmine blast in Afghanistan in 2006.
He suffered more than 40 injuries, including brain damage which affected his memory and speech.
He has since defied medical opinion by teaching himself to walk again for short distances using crutches.