My 7-year-old son is similar size (54" tall, 72 lbs.). He rides a 108 cm board. Each manufacturer usually has their own size recommendations, primarily based on weight, but I'd say in general your son would probably do well on a board anywhere between 100-120 cm. The shorter end of the range is probably best for learning. I'm guessing by LTR board you're referring to the Burton LTR series? I don't think they sell them new to the public since it's their rental line, but you can find used ones online. The 108 my 7-year-old uses was a hand-me-down from his older brother, but I got a 95 cm board for my 6-year-old off craigslist for $40, with bindings and boots included, so that might be another option if you don't want to rent but want to keep it cheap. Just do some searching to get familiar with pricing so you don't get ripped off. This might be a better bet if you have younger kids as well.