Welcome Memedixie. I'm sorry for your struggles with your son.
There have been many stories on this site of parents who have turned their adult difficult children in to the police for stealing from them. It's a horrible experience for parents and yet it is the correct response to theft if we want our kids to learn the consequences of their behavior. That doesn't make it any easier on us though.
Since your son abuses drugs, I'd strongly recommend you attend an Alanon 12 step group, many parents here find solace and support there. You can also post on the Substance abuse forum here, the parents there are well versed in substance abuse. If your son has mental illness issues, then you might try contacting NAMI, the National Alliance on Mental Illness, you can reach them online and they have chapters in many cities. NAMI offers excellent courses for us parents which help with resources, information, guidance and support. I'd encourage you to seek as much support as you can. As you mentioned, in addition to the loss of $2500, you are dealing with the betrayal as well, and that can be more difficult then dealing with the theft.
At the bottom of my post here there is an article on detachment, it is very helpful.
It sounds like you've been at this awhile with your son. It is mentally, emotionally, physically and often financially exhausting having to deal with adult kids who go off the rails for whatever reason. Turning your son in is not an easy choice, but it sounds like it is the right choice. That choice causes much suffering for us parents, so my advice to you is to avail yourself to as much help as you can.
You deserve to have peace of mind as you approach retirement age. I am in my 60's and have a 43 year old daughter who I've had to work hard to learn to detach from.......or I would have slipped down that dark rabbit hole with her and literally ruined my own life too........detaching from her has been one of the hardest things I've ever had to do and yet, for me, it was the right thing to do.
We are powerless to alter the course of our adult children's lives, we can't fix them, change them or control them. But we can enact strict boundaries to protect ourselves and we can demand respect.
Keep posting as you go through this, it's very helpful. I'm glad you're here, we understand the pain you find yourself in......you're not alone.