The problem turned out to be in my datasource setup in JBOSS_HOME/standalone/configuration/standalone.xml.
In the validation section, I had this which was incorrect.
<validation>
<check-valid-connection-sql>select 1</check-valid-connection-sql>
<validation>
The above was incorrect and DB2 was giving a validation error.
I changed that to the following instead:
<validation>
<valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.db2.DB2ValidConnectionChecker"/>
<stale-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.db2.DB2StaleConnectionChecker"/>
<exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.db2.DB2ExceptionSorter"/>
</validation>
Here is the complete datasource set up in case anybody else finds it useful.
<datasource jndi-name="java:jboss/datasources/DB2_zOS_DS" pool-name="DB2_zOS_DS" enabled="true" use-java-context="true">
<connection-url>jdbc:db2://MyHostName:MyPortNo/MyDBQA1:currentSchema=MySchemaQA1;</connection-url>
<driver>db2zOS</driver>
<transaction-isolation>TRANSACTION_READ_COMMITTED</transaction-isolation>
<pool>
<min-pool-size>9</min-pool-size>
<max-pool-size>50</max-pool-size>
<prefill>true</prefill>
<allow-multiple-users/>
</pool>
<security>
<user-name>MY-APP-ID</user-name>
<password>My-APP-ID-PASSWORD</password>
</security>
<validation>
<valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.db2.DB2ValidConnectionChecker"/>
<stale-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.db2.DB2StaleConnectionChecker"/>
<exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.db2.DB2ExceptionSorter"/>
</validation>
<timeout>
<set-tx-query-timeout>true</set-tx-query-timeout>
<blocking-timeout-millis>500</blocking-timeout-millis>
<idle-timeout-minutes>15</idle-timeout-minutes>
</timeout>
<statement>
<track-statements>false</track-statements>
<prepared-statement-cache-size>32</prepared-statement-cache-size>
<share-prepared-statements>true</share-prepared-statements>
</statement>
</datasource>
<drivers>
<driver name="db2zOS" module="com.ibm.db2">
<driver-class>com.ibm.db2.jcc.DB2Driver</driver-class>
</driver>
</drivers>