Change Management roles in small IT organization

Answer:
There are several points I'd like to address related to your situation:
1. Initiator - that could be user, customer, someone from IT department, supplier/vendor, CSI, Problem Management, Service Desk...so, it's not necessary that only someone from Change Management i.e. IT department triggers the process.
2.Assessor - that depends on the change category. E.g. for standard changes (see the article Tips and tricks for using the ITIL standard change mechanism https://advisera.com/20000academy/blog/2017/06/27/tips-and-tricks-for-using-the-itil-standard-change-mechanism/), you don't need assessor. See the article "ITIL V3 Change Management – at the heart of Service Management" https://advisera.com/20000academy/knowledgebase/itil/ -v3-change-management-at-the-heart-of-service-management/ and "Three key elements of assessment and evaluation of changes according to ITIL" https://advisera.com/20000academy/blog/2015/06/30/three-key-elements-of-assessment-and-evaluation-of-changes-according-to-itil/ to learn more about other types of changes
3. Approver - that's related to assessment and types of changes. Depending on the type of change - so will be the authorization
4. Change Manager - see the article ITIL/ISO 20000: What is the job of the Change Manager? https://advisera.com/20000academy/blog/2016/10/04/itil-iso20000-what-is-the-job-of-the-change-manager/ to find out more

It is usual (particularly for smaller organizations) that several roles are combined in one person. This article provide more information "What ITIL roles can be combined in one person?" https://advisera.com/20000academy/knowledgebase/itil-roles-can-combined-one-person/