I don't want to buy any synthetic teethers/soothers. Any suggestions on homemade/natural teether or soother?

If baby is 6 months and above, you could let him chew on cold cucumber and celery sticks. The cooling properties of cucumber help to sooth baby's gums. Having said that, my girl does not like cold food. She finds them weird. Haha. So I give it to her at room temperature but it also works well.