Being a premature baby and at 1 year age able to stand with support is a good sign. It's normal.
Coming to gross motor milestones for a normal child at 1 year age are 1) Should be able to sit with or without support 2) should be able to stand with or without support.
Next Steps
Watch for his growth and development. Give all vaccines as per schedule.
Health Tips
All developmental milestone have range of age period. Sometimes may also get delayed. There is always some variation in each and every child. Nothing to worry.