Stuart McGowan, 41, of Kennedar Drive in Govan, was found guilty of embezzlement this week at Glasgow Sheriff Court.
But the family of his 25-year-old victim say they have been robbed of justice after he was sentenced to community service and fined £2000.
They believe the loss was greater than the £2200 McGowan was found guilty of taking.
The victims' mother referring to McGowan said: "His act of wilful and sustained dishonesty has had a devastating effect on us as a family.
"He held a position of trust in both the organisation he worked for and within our family as we had contact with him on a daily basis and trusted that he would look after my son's best interests with integrity, respect and compassion.
"His dishonesty tore that trust apart and plunged us into a world of mistrust and doubt.
"I feel he has not been sufficiently held to account for his actions."
McGowan is understood to have been sacked by his employers, Key Community Supports, in Glasgow, after he admitted stealing from the 25-year-old, from Glasgow, who also has autism and requires 24-hour care.
But his family say McGowan handed himself in only after concerns were repeatedly raised about missing money.
His mother said: "The Fiscal addressed Mr McGowan and stated that this was 'a very serious crime' but I fail to understand the community payback sentence, which amounts to eight hours per week over six months.
"It was quoted in his defence that he turned himself in and this was seen as a positive step but the reality was he did this only after I had raised my concerns frequently and they were eventually investigated by the company employing him.
"I feel this was seen as a victimless crime as my son's autism and learning disability prevent him from knowing the facts. This man is now employed once more.
"Many vulnerable people need to be aware that this is happening and if it stops even one family from going through the heartbreak we have over these past 18 months it is worthwhile."
The court accepted McGowan's plea of not guilty to other charges of theft and fraud. During the hearing, he blamed the effects of the death of his brother and breakdown of his marriage for his crime.
He was initially given 250 hours of community service but this was reduced to 200 hours. He is also entitled to pay back the £2000 fine at £20 per week.
Another family member said: "I'm absolutely disgusted at this."
A spokesman for Key Community Supports declined to comment.
McGowan was unavailable for comment.