No, he cannot automatically keep the security deposit. However, he may be entitled to some or all of it, since it’s a fund to offset damages that an owner may incur because of a tenant’s default under a lease.
The Texas REALTORS® Residential Lease (TAR 2001) lists the deductions that the owner can make from the security deposit because of a tenant’s breach of contract of the lease. In your case, it’s likely the delinquent rent and other damages you describe will exceed the amount of the security deposit and the owner will be allowed to retain all of it.
Since the tenants left a written statement of their forwarding address, you or the owner are required to give the tenants a written description and itemized list of the deductions from the security deposit within 30 days after the tenants surrendered possession of the property.