...although I arrowed one last year around the first of May.  My son is going to join me this year on our farm in Eastern Montana, but he needs to make it the first weekend of the season (April 15-20), due to a pending grandson.  

My question is whether the toms will be as active that early or not?  Thanks in advance for your advice.  We are going then anyway, but kind of want to know what to expect.

Any chance you can get out and do some pre season scouting. Getting out early morning and listen for gobbling activity. Finding where the birds are headed will give you a good start.

In my very limited experience the toms have been very active at the start of our season which starts about mid April.