Lantus lasts ~18 hours, so if he's going high in the evening after a lantus dose at 9pm the day before, that could be attributable to the lantus running out. Many people will split their lantus dose to tackle this, taking some at bedtime and some on waking. But the starting point to determine whether lantus is the issue is a basal test, or a few days of afternoon/evening basal testing.
The other thing I'd mention, as a note of caution, is are you basing the evening highs up to 18 purely on the libre or are you finger pricking to confirm? Libre, or any CGM, isn't 100% accurate and the loose their accuracy and the higher/lower ends of the scale. So if you finger prick you might find he's 12 and because, hopefully, your libre doesn't see 12 very often all it is seeing is high and it shows you 18 - when basal testing or determining dose adjustments use finger pricks for certainty.