Let me clear some confusions.
1. Conditional PR. Until recently, when a spousal PR application was approved, the spouse used to get full-fledged PR. It could be revoked only in extreme circumstances. This resulted in many "marriages of convenience" where people married Canadians just to get PR status and then soon after they arrived, they abandoned their Canadian spouse. Hence, CIC introduced "Conditional PR" for spousal PR application. Under this process, a conditional PR is issued to the foreign spouse. Only after the spouse completes 2 years of married life (or has a child) would the PR be deemed as full-fledged. If in the meantime, the foreign spouse breaks the marriage, the PR becomes void.
In your case, since you are already married for 2 years, the conditional PR would not be applicable to you. (Even if it was less than 2 years, if you had a child, that too would grant full PR for your spouse).
2. Residency requirement for maintaining a PR status. Once a person gets PR, they have to maintain their PR status. This is done by staying in Canada (or be able to stay in Canada) for at least 2 out of the last 5 years. The PR can leave Canada at any point in time. But at the time of returning back to Canada, (s)he should be able to prove that they have met this residency requirement or would be able to meet the residency requirement. (For example, a new PR comes to Canada and leaves immediately. When this PR returns after one year, (s)he still has another 4 years to complete the 2 years of residency requirement). If the PR leaves Canada to stay with a Canadian, then this stay is counted towards the PR residency requirement.
There is one more thing to note: To be able to return to Canada, the PR must carry a "PR Card". Without this a PR cannot enter Canada via air/sea.
3. Residency requirement to be eligible for Citizenship. To be eligible to apply for Canadian Citizenship, a person must stay in Canada for 3 out of the last 4 years. The only exception here is if the person stays outside Canada because the person's spouse is a Canadian and works for Govt/Military. (Canadians working in pvt sector abroad is not included here-unlike in PR residency requirement)
To sum up your specific question, irrespective of the length of your marriage, your spouse can leave Canada to visit his mum in Europe right away. However, he should ensure that he has his PR card with him when he returns.
