بسم الله الرحمن الرحيم
In the question mentioned above when you gifted 300 gram gold to your minor son out of 800 gram gold then you have to pay the zakah of 500 gram gold only.
Note: However, if you mean something else with the word ‘waqf’ then please explain it and submit your question again.
Allah knows Best!
Darul Uloom Deoband