First, it's important for you to understand how to use Lantus. Lantus craves consistency so you need to find a dose that you can give both AM and PM without dropping him too low.
Every time you change or skip the shot, you disrupt the depot that has to build which is why Lantus works as well as it does. The dose should be based on how low it takes him, not the Pre-shot. The Pre-shot is only to make sure he's high enough for insulin at all.
Now with SLGS, until you get more data, if he's under 150 at Pre-shot time, you should skip.
Also, it's important for you to be getting tests on every cycle if you can. A curve once a week really just isn't enough information to be able to make good dosing decisions. IF possible, you should try to get at least 1 mid-cycle test on the AM cycle and at least a "before bed" test on the PM cycle. Most cats go lower at night so it really is important to get at least that "before bed" test.
If that 90 was a valid test tonight, I think it might be best to drop his dose down to .5 and then hold it (and get those extra tests in when you can) because if the 1U dose he got this morning dropped him that much and you skipped the dose last night, 1U may be too much.