Without knowing what his numbers look like, it's impossible to make any recommendations about changes and stopping those meds.
I would also disagree about Warfarin/Coumadin- it can be a pretty 'hardcore' medication if not monitored carefully, increasing risk of bleeding and other issues. I'm assuming that he is being properly monitored.
How well is diabetes controlled? Do you have his A1c or fasting/post-meal/random blood sugar readings? I'm going out on a limb because if the combination of metoprolol and warfarin and guessing he has atrial fibrillation? If so, strenuous exercise can be dangerous in some patients because of the inability to control the heart rate. Do NOT try to get him changed to Rosiglitazone, those meds have cardiac issues and can cause/exacerbate heart failure(though that one is the least likely to cause them). I'm guessing he's probably also got the beginnings of heart failure, too, considering the Afib? How well is his thyroid issue controlled? Too much Synthroid can cause cardiac arrhythmias, too little can make it nearly impossible to lose any weight. Because of the first part, you have to be conservative with the dosing because of his other disease states.
I'd also suggest that he needs to be on MORE meds. Every diabetic that can tolerate ACE inhibitors OR ARBs should be on one, for kidney protection. Every diabetic, regardless of current cholesterol numbers, should be on a statin. There is clear evidence-based data to support this.
He needs to talk to his doctor about starting diet and exercise. These two interventions will do more than any medication to improve the quality of and increase the length of his life. Do not start an exercise plan beyond light walking without discussing with his doctor because of the Warfarin and Metoprolol. Do not start a new diet and introduce a ton of veggies immediately to his diet, because of the Warfarin, because it can affect his bleeding risks and will need to be monitored for that while he is changing. That being said, he should (after talking to the doctor) shoot for 30 minutes of walking 5 days of week and exercising with weights twice a week.
Good luck, I hope your dad is willing and able to make some changes in his life. Diabetes is a nasty disease, but it's one that can be controlled if a patient is willing to make some life changes.
I'm going to reiterate again- get your dad to talk to his doctor before starting anything major. Too much too fast can be dangerous. Go with your dad if you can. Sit down with him and tell him you love him and want him around for awhile. Most people will not do this for themselves (obviously) but will make hard choices for the people they love because of the way their actions can affect their loved ones. Be supportive, understand that it's a long road and there are going to be bumps along the way, and that it's not as easy to lose weight and get healthy when you're 60 as it is when you're younger.