Does my son-in-law still have a share in the land I bought with company money after his verbal waiver of his share?

Your partner's relinquishment of his share in the company in exchange for his capital is permissible and binding, as long as it was done with his consent.

If the company's capital is still intact, he has the right to claim it and waive his share of the profits.

However, if the capital has decreased due to losses, he does not have the right to claim it unless you agree.

A verbal agreement is legally binding in Islamic law; written contracts are for documentation and confirmation, not for the validity of the contract itself.

Your brother-in-law has no right to share in anything with you after his initial relinquishment, unless you release him from that relinquishment.