The two primary consideration here are the dogs safety and the ease (or budget) of improvement.
There are several options, but the surest and safest are physical restraints either a fence or an overhead cable run. Fences are more expensive require more time and energy to create. Fences are also not an effective restraint for an unsupervised dog. A dog left in a fenced yard will dig under, or chew through the fence after sometime.
The dogs dislike of the overhead cable run will be significantly less than the dogs dislike of the getting hit by a car, if he escapes. There are safety concerns with a cable run that should be fully considered.
There are some solutions where a post is sunk into the ground, and a chain is allowed to circle around it. The screw in the ground type sold at most pet stores are not appropriate for a big dog, as they can pull them out, and are also not appropriate for long term unsupervised stays as they can and do get tangled.
For unsupervised outside stays of an hour or so, a fence will provide reasonable safety. Assuming that it is checked regularly. For unsupervised stays exceeding an hour a cable run can be the safest choice.
For supervised outside visits a post and circling chain can be used. In my childhood we tried the store bought screw in post on a big dog (with mixed results). Later we used a modified version, the rear axle (internal part of a solid rear end) was driven into the ground about 2 feet. The chain was attached so it would spin around the axle.
There is also a temporary heavy duty solution available from feed stores. 16 foot long fence panels that are 4 feet high. They have small openings at the bottom and larger at the top. Different stores have different options, but here is an example link I like the kind with the 2 inch opening at the bottom, and 6 inch at the top. Have used them for everything from dogs, to pigs, to goats and cows. Anchor them with steel posts, or tie them to trees or each other. Very portable and long lived.