Tā de chéngnuò zǒngshì yào dǎzhékòu, bù néng wánquán xiāngxìn.
His promises always fall short; you can't fully trust them.
Tips
usage
打折扣 has two senses: the literal (giving a price discount, e.g. 打八折扣 = 20% off) and the figurative (something failing to meet full expectations, e.g. 大打折扣 = greatly diminished/fell well short).