Lantus is a depot insulin which works best with consistency and it can take a good 3 days to see how a dose is going to work out. That's because it builds up a depot or storage "shed" of the insulin which is gradually used. You change the dose based on the nadir, usually around +6 hours after the insulin. You want any changes to be small and slow, with enough time in between for the full effect to be observed and to change back if the effect is negative.
You had 3 days at 1.5 units and it resulted in not being able to give insulin safely; thus, I would back off the dose.
Go back to the 1.25 for a minimum of 3 full days (6 cycles) unless he again goes too low to shoot (looks like you've set 150 as your cut-off; normally, we recommend 200 for new insulin users until they know how their cat handles it)
On the 3rd or 4th day, check the nadir at +6 and follow the guidelines posted in the Tight Regulation forum. (And you're welcome to post there, now that you've got your profile, spreadsheet, are blood testing, etc)
Also, the low AM pre-test could be a number of things. You need to check for why that might be - was insulin too high? were there food changes? activity changes? fall off in appetite? so that you can make the best decisions for Benjamin.