It is fantastic that you are working with your dad to improve his level of lifetime fitness!
First, your dad is young and there is no better time to see amazing results!
Second, make sure his physician has given the ok for training, especially in any areas that have prior injury. If necessary, see a Physical Therapist for a full program of home rehabilitation exercises that you can use for warm up and cool down as well as for some specific training in injury areas.
Should he have protective gear in his work to protect his knees? I think if he is on them, then the answer is yes and it should be non-negotiable.
While it would be ideal if he switched from beer to say green tea, perceived quality of life and the perception that we have to "give something up" or "suffer" may make compliance a serious issue. For things like that, you want to give him options and make them easy for him to enjoy, but let him make any decisions. In truth, the beer may have contributed in a number of ways and some of what you want to try to do is change the effect of this on his body.
One thing you are aiming for is to change the chemical environment in his body. Right now, he does not have the right chemistry to get optimal results from any training.
So, this is a transition period. Lets say you think of this first period as a 6-8 week transition period.
Now, starting of with low weights and slower movements has a few advantages.
He needs to strengthen the structural components of his physique as well as change the chemistry. This time will help him train tendons and ligaments as well as muscle. Additionally and perhaps more important, he will remodel his neural responses as he learns to model proper movements and form!
In short, your general approach is good!
What I would be sure to do...check him for any areas of pain, for any structural imbalances, assess him for any lingering injuries which might make certain movements uncomfortable or unsafe. DO not do any movement that look like the movements that have caused any over use injury.
His knees hurt? So select movements that don't hurt them.
But more important, why do they hurt? Is there a muscle imbalance, an injury, is there a calcification in nearby tissue that impinges on the movement...
Just doing movements that are 1/8 squats or partial lunges
with knees tracking over the toes and not extending forward of the line of the toe, might be better for him.
Use a rack to start lifts instead of squatting to pick up the bar.
Lying squat machines can also be good for this. Try different things to see where there is no pain.
Watch him do a movement. Do the knees track left or right, or do they stay centered over the middle toe. Consistent movement to the side implies there is a possible muscle imbalance. In general these can be corrected by relaxing the tighter muscle (the one on the side the joint pulls to) and isolation training the weak muscle (the one on the side the joint pulls away from.)
Key areas to watch for imbalance...
Neck, Lower Back, Shoulder, Hip, Ankle, Knee
Diet is important!
- He needs good fats to combat swelling from poor diet choices and from injury (3-6 grams of Fish oils a day for instance). Fat is also a precursor for growth hormones and we want to see those levels shoot up in the next 2 months of transitioning.
- He needs a variety of good quality vegetables (anf fruits) to help buffer acid buildup and provide phytonutrients and antioxidants.
- He needs enough unprocessed qualiry carbs to give him energy for training and recovery.
- He needs enough quality and timely protein to insure uninturruted recovery.
Glucosamine (& chondroitin) is also something to consider. You take it for life. 1500 mg/day Glucosamine is the effective dose in research for joint related issues. For example, I use this:
Glucosamine and Chondroitine
for joint repair and health as well as tissue repair.
Make the workouts fun adn short. ALWAYS warm up and cool down. (Try a walk before adn after to get the heart rate up/down) Make the food choices easy and enjoyable. Make the training interesting and safe, but a little challenging.
In 6-8 weeks, change the training to include a progressive component that includes working up to some of the basic maximal lifts. Get his muscle and strength WORKING for him.
Once you get there, except for any special considerations, he is just like any one of us.
Training, diet, recovery, lifestyle, progression.
Good luck!