My question is, how do I find out where all the places he had a 401k plan at, and how do I go about contacting them?
Another way to check for whether he participated in a 401k, is to check prior years W-2s in box 12, assuming you can get access to these. A couple of other resources:
https://www.unclaimedretirementbenefits.com/ - nationwide, secure database listing of retirement plan account balances that have been left unclaimed
https://www.efast.dol.gov/portal/app/disseminate?execution=e1s1 - Plans are required to file a tax filing called a Form 5500 every year. You can enter the company information and see if they filed, and then on the filing will be the plan contact's information. This can be helpful if you know the company he used to work for was sold to someone else, but you're not sure who to contact.
You will have to contact his former employers to verify if he was participating in their company sponsored 401(k) plan. If he worked for large corporations, then the HR and benefits departments are good places to start. For smaller organizations, the business owner or office manager often handles the administration of 401(k) plans. If you do not know all of his employers, then contact the social security administration, or look at his most recent statement from them - it contains employer histories. Hopefully, your brother designated his beneficiaries properly or has a will to determine the distribution of those funds. Best wishes!
I'm very sorry about your brother...wishing you comfort and peace in your loss.
You may have to do a fair bit of "detective work" regarding your brother's financial affairs because unfortunately there's no centralized repository for orphaned 401(k) accounts.
Probably the first step--IF you are authorized (see below) and have access--would be to review any financial records your brother may have kept. You might find past 401(k) statements with information on the companies he worked for and a phone number to call with questions. You might also find information on other accounts and insurance policies that you didn't know he had.
The distribution of any 401(k) accounts your brother had will largely depend on the beneficiary designation he made on those accounts, if any. So if that information is not available in his files, it's one of the questions that will need to be asked of the Human Resources office or 401(k) Plan Administrator at your brother's former employers. In all likelihood, you won't get any answers from these parties unless you are the legal representative for your brother's estate (see below).
This July 2013 Denver Post article has a few more 401(k) location tips from Rick Meigs, president of the 401khelpcenter.com: http://www.denverpost.com/ci_23563540/tracking-down-old-401-k-accounts-is-tricky
Also, you'll need to know that, even though you are the only remaining family member, from a legal standpoint that doesn't automatically mean that you are authorized to handle your brother's estate affairs.
If your brother left a Will, he likely would have appointed a Personal Representative (Executor). And if that's the case, then the Personal Representative will be responsible for handling estate matters, including locating 401(k) accounts and making distribution arrangements.
If your brother did not leave a Will--and if his estate needs to go through probate--the court will appoint a Personal Representative (or Administrator) to handle estate matters. And it may be that you're the logical choice, but it's not automatic.
This is just general information to get you started. Be sure to consult an estate attorney for specific advice if it turns out that you are the one to handle your brother's affairs.
Hope that helps.