My son weighs 45kg is will nurofen syrup be effective for fever 38.00?

Yes but would not do. Nurofen will bring it down but it is unwise.
FEVER IS HOW OUR BODY FIGHTS INFECTION!!!
It is the key to our immune response and by lowering it you will sabotage his immune system and prolong his illness. 38 is barely a fever and I would not try to lower unless it goes to 40. Keep him well-hydrated. Give a bit of Nurofen only if he's really miserable.
See https://tinyurl.com/y7a84ph8.