Yes

If the grandson is not a legatee named in the Will then he has no locus whatsoever to file such a suit

APPLICATION UNDER ORDER 7 RULE 11 OF CPC FOR NO CAUSE OF ACTION AND IF THE PARTITION DEED WAS REGISTERED THEN ADDITIONAL GROUND OF SUIT BEING BARRED BY LIMITATION BECAUSE KNOWLEDGE ABOUT PARTITION WILL BE COUNTED FROM THE REGISTRATION DATE WHICH IN YOUR CASE IS BEYOND LIMITATION PERIOD FOR FILING SUIT

Deed of partition duly stamped and registered would be binding upon legal heirs 

suit can be filed after period of 21 years 

it has has to be proved that grand father executed the will 

See if the legal heir is no way beneficiary to will then he has no cause of action to challange the partition mutual made by all the legal here and the suit is not maintainable. 

The Will and its execution have to be proved by the claimants. Their identities as beneficiaries under the Will should be proved beyond doubt. 

Claim for possession of immovable property or any interest therein based on title becomes time barred by 12 years from when possession of defendant becomes adverse to the plaintiff. 

This actually means a period of 12 years + 12 years for possession to become adverse in normal cases.  These are further subject to following conditions:

Where the knowledge of right on title is concealed by fraud of others, the period of limitation shall not begin to run until the plaintiff has discovered the fraud or the mistake or could, with reasonable diligence, have discovered it, or in case of a concealed documents, until the plaintiff  first had means of producing the concealed documents or compelling it's production. 

He can challenge that but you can object the same to be time barred suit

If he proves in court that he had knowledge of the same in 2018 then it will definitely not be a time barred suit but if not then it will be dismissed on limitation. 

Hi, you can challenge the suit at initial stage ,and it can be dismissed on considering of facts 

If a person does not protest someone illegally occupying his property for 12 years, then the squatter would get ownership rights over that property. 

When WILL bring in knowledge of beneficiary ? Declaration suit will file along with cancellation of partition deed and recovery of possession.

Claiming any property based on a Will after a gap of two decades or more may be barred by limitation especially when ther was a partition already done .

However let him file a petition to probate the Will, once the court is granting probate then he can file a suit to take possession the properties now distributed among the legal heirs.

1. There has been a limitation period of four months for the execution of the Will after the death of the testator.
2. But, there are cases wherein the beneficiary came to know about the Will after so many years like mentioned above.
3. Then the other party who is denying the will that the beneficiary had the knowledge of the Will, but didn’t turn up for so many years.
4. Rest, the possession should not be left until and unless the person challenging the partition, proves the invalidity of the same before the court of law.