There are 2 options here:
1) Have the previous colleague add you as an admin in the dashboard where the org is
2) Assuming you have created a new org for yourself, have the previous colleague remove the devices from inventory in the old org
If neither of these can be done, you may need to try to get access to his email where the dashboard account is. Depending on the situation make a legal notice to him and see if he will make the moves needed.