Wow, your Dad and Grandfather both have trees they have had since they were teenagers? What the hell you asking a bunch of know nothings like us about Bonsai, you have 2 very experienced relatives to bend their ears. Damn, wish I had someone in the family pass down a tree or two to me. Seriously, tap their knowledge as much as you can.
You are in the Portland area. All kinds of trees grow very well there, too many to list. Take a walk in the mountains and collect a few. Best way to get a tree that looks like it is a century old in a pot is to start with one that is already 2 centuries old. Get out there and collect while you are young and healthy enough to do some ridge walking and mountain hiking.
Bristlecone pines have a mystique, and the very few older examples in cultivation have made good bonsai - but it has a HUGE downside. It is so slow growing, you will need 50 years to get the growth you would get in 10 years from a Limber pine or most other pine species. Bristlecone only has a single flush of growth, and that flush is of short duration. They often go years without adding any growth at all. They prefer an alkaline soil, add crushed oyster shell or crushed limestone to your potting mix, which should be pretty much all inorganic. They are tricky to keep healthy. They need intense, full sun, like you get above 10,000 feet, cloudy Portland will result in soft growth that is more fungus prone than most pines. They hold old needles for as much as 30 years, and if you remove needles before they naturally drop, you stress the tree. So they always have a dense foxtail look. Really a tough species to do well. Well worth the effort, especially if you collect one with the first 50 years of growing out of the way. Most of their native range is protected, so finding a legal area to collect bristlecone is tricky. BML land is usually easy to get a collecting permit, less than what ever number their limit is, for personal home landscaping use, is what you ask for when obtaining a permit. I think the limit is 10 plants. Ask at local BLM office. Similar for State Forest. Each office will tell you what areas you can collect in. But if you are only after Bristlecone, you will be above 10,000 feet for much of your hunt.
Look at your local state DNR for a list of pines native to your area and your elevation. Shore pine and Lodgepole pine make excellent bonsai, and I think one or the other is local to your area. They will grow fast enough you can actually train it for bonsai. Ponderosa pine, Limber pine and Southwestern white pine all make good bonsai and are much easier to handle than Bristlecone pine. Limber pines live for 500 to 1000+ years, so only a quarter the age of a Bristlecone, but so much easier to handle. In Portland area, Japanese white pines do reasonably well, a very traditional bonsai pine. You might be on the cool side in summer for Japanese Black Pine, but Michael Hagedorn who is in the Portland area certainly does a good job with JBP.. The possibilities in your area are near limitless in terms of easier to grow pines.
And if you want to branch out, there are several spruces native to your area, Engleman Spruce make excellent bonsai. Mountain hemlock is another possibility, but Mountain Hemlock will not do well away from the Pacific Northwest, so if you think your kid might move to some other part of the country, Mountain hemlock might not be the best choice. Firs also make good bonsai, sub-alpine fir has some of the draw backs of Bristlecone, in that it is slow growing, in addition it does not tolerate warmth, especially at night. Needs to have temperature drop sharply at night, like it does in the sub alpine areas it is native to. But other firs work. And in Portland area Coast Redwood is always an option. They make good bonsai.
So lots of choices for your legacy tree. If you really want to stick with Bristlecone pine, go for it, but be warned, they are problem trees. They are great, when done well, but are a bitch to do well. Start a Lodgepole or Shore pine, along with your Bristlecone. If the Bristlecone dies on you, the other can be your spare that you started as a teenager, to pass on.
Actually start 5 or 10 pines, one or two of each of your local species, start them all before you turn 20. All bonsai hobby growers have a certain amount of mortality. If you start 10 now, chances are better you will have one still alive when you turn 45 and your kid turns 18. We all have made mistakes and killed a few trees, it is a steep learning curve, best to have a few spares on hand in case you kill one or two. Lord knows I've killed too many to count over my 40+ years of doing bonsai.
Good luck and show us photos of the tree you Dad gave you.