Get ready for a bumpy ride... things are complex. (I'm also providing some info on Severance here, since I saw you asking about that in another thread).
1. Severance (the old RFRG): Since your husband is releasing (and he must release; a transfer to the Supp Res will not trigger pension entitlements), the severance is a "retiring allowance" under the income tax act. Therefore, for all years prior to 1996, you may put $1500 into RRSPs above and beyond your normal contribution room. If he enrolled in 1986, that makes 10 years or $15K that can be rolled into RRSPs above your available contribution room. Ref: http://www.cra-arc.gc.ca/tx/bsnss/tpcs/pyrll/rtrns/t4/spcl/trnsfr-eng.html
2. For Reservists in part I of the CFSA, like your husband, there are two numbers that are important: Years of pensionable service, and days of CAF service. A year of pensionable service is a year enrolled in the plan. That year can include both paid and unpaid days. The benefit is calculated based on Days of CAF Service. Thus, any years where your husband was enrolled in the CAF but not working full-time, he does not receive credit for days he did not work. So a benefit calculated on 25 years out of 30 years is not, om its face, unusual. Ref: http://laws-lois.justice.gc.ca/eng/regulations/C.R.C.,_c._396/page-7.html#h-18
3. Average salary for a Reservist in Part I is calculated differently; there's a formula in the Superannuation Regulations. Annualized earnings are calculated for each calendar year of service. Simplified description: Take pay earned in the year, divide by the number of days worked in the year, and multiply by 365 to get the annual salary for the year. The best 5 consecutive years of annual salary are used to determine the benefit. Ref: http://laws-lois.justice.gc.ca/eng/regulations/C.R.C.,_c._396/page-7.html#h-18
So, assuming you husband does re-enrol after a year away, and assuming that he is not absent for one full calendar year, then as long as he has some earnings in each calendar year he will have his earnings for those years included in a future benefit calculation.
Note that if your husband does re-enrol he cannot buy back the year away. No service = no buyback.
He will have to meet the enrolment standard for the CAF, the same as anyone coming in off the street; serving members who no longer meet the enrolment standard or their occupational standard can be retained; once they're out, they are a new entrant, like any other person. There is no guarantee of re-enrolment or resumed full-time employment.