applying for citizenship after 9 month absence from UK?

My USC OH and I have been married 10 years, 6 of that was spent in the UK. OH has an indefinate leave stamp in his US passport....but somehow in 6 years never got to taking the test and aplying for citizenship....now we've been in the US for 9 months and are returning to the UK next month...here's my question. I know in theory he doesn't need UK citizenship ,but let's say he wants it, does he need to wait now or can he apply straight away and the 6 years spent here previously will count...?
Thanks

I think he'd need to be back in the UK for a year.

As a spouse, he needs to reside in the UK for 3 years, but the last 12 months of that 3 year period, he can't be out of the UK for more than 90 days...