My questions are:
1. can i challenge the partition now, if so on what grounds.
Since you all the three decided to partition the property in the maner mutually agreed among you three and have prepared partition deed, got it registered and have been possessing and enjoying their own respective share of property for more than two decades, it will not be maintainable to seek for re-partition at this stage
2. can my son challenge the partition.
Your son has got no rights in the property especially during your lifetime
3. can i challenge the will.
The will was made by your mother in respect of her own share in the property, hence it cannot be challenged now.
Since your mother had marketable title to her share in the property, her will is valid and legal.
No dispute on this shall be tenable in law.