His weakness could well be due to the steroids. - but to be honest, at 88 and after having PMR it is hardly surprising! At 60 with a serious flare of PMR behind me plus I had been on crutches for 9 months due to an inflamed achilles tendon - I seriously struggled with even a few steps and a flight of stairs defeated me on many occasions. The difficulty your father has with slopes may be associated with the ruptured achilles tendons because that affected my ability to walk up hills - and I was told I must NOT do so whilst the inflammation was still present. It was due, by the way, to being put on a quinolone antibiotic whilst on pred. It is a known but unusual side effect and noone with a history of pred usage should be put on a quinolone and this is particularly so if you were on Medrol (methyl prednisolone). It is only relatively recently that slopes have become easier - nearly 2 years with a daily half hour brisk walk!
There are no real alternative treatments for PMR. Pred is the gold standard therapy and if it is just PMR works well. Did the GP suspect he was developing GCA when he had him on 60mg - there is no necessity for a dose that high in PMR, 15-20mg/day is the recommended starting dose. This is not least because only PMR responds so well to a moderate dose like this - other things (such as other inflammatory arthritides) may respond partially to higher doses and that confuses the diagnosis. About 1 in 6 patients initially diagnosed with PMR have their diagnosis changed at some later point, mostly to another form of inflammatory arthritis and most often to LORA (late onset RA).
Methotrexate is NOT a treatment for PMR. The role of MTX in PMR is that it changes the way the body metabolises pred and is thought by some to mean you get the same effect from a lower dose of pred - it is classified as a "steroid-sparer", there are one or two other drugs used in the same way. All are normally used in rheumatoid arthritis (RA) but none have really shown a reliable effect. Three studies were carried out in the past with MTX: one showed it helped, one showed it didn't, one didn't come to a conclusion. I know people who have been put onto MTX who were initially able to reduce their dose of pred considerably but then had major flares, one developing GCA, and had to go back to 15mg and even above. They still wonder whether they would have had the flares had they been on a slightly higher dose of pred because MTX can never replace the pred and only reduces the dose by a small amount, if it does at all. And it has some quite unpleasant side effects for some people, know as the "MTX hangover".
It does appear that in some cases where MTX, azathioprine and other RA drugs appear to work in lowering the pred dose, that it wasn't PMR but another inflammatory arthritis - it is that dramatic response to a moderate dose of pred that is the sign. When that isn't there the question has to be "Is it PMR?"
One drug that showed promise in a pilot study with 23 PMR problem patients was leflunomide - 22 of the 23 achieved remission of their PMR. It is used on occasions where there is a real need to get the patient off pred for various reasons but if it is going to demonstrate side effects then they are dramatic and can be life-threatening.
If I were your father (or is it you who is concerned?) I'd accept the current status quo, especially if he can reduce the dose again somewhat using a slow reduction - there is one on the forum that has worked very well for a lot of people. At his age it is possible he may need a low dose for the rest of his life but it isn't going to do much damage. Once a patient is down to about 8mg they are in the "physiological range", the amount the body would make anyway.
And for going out - get a mobility scooter. I had decided that if things didn't improve a lot within a few months I was getting one. That was at 60. Occasionally when I look at a rather more demanding walk up slopes I wish I had done so! There is a very small folding one that will fit in any car boot, even a Smartx2!