He began his second season with the Portland Sea Dogs, looking for progress and an eventual promotion.

But on May 13, Tate Matheny  was batting .168 with one double.

Since then?

“He’s made some adjustments,” Sea Dogs Manager Joe Oliver said.

Matheny showed off his improved swing with a two-out, walk-off grand slam Monday afternoon in a 7-3 win over the Altoona Curve before 7,040 at Hadlock Field.

“Just trying to find a pitch I could drive,” Matheny said. “Luckily, I got a pitch over the plate.”

Until two weeks ago, the only driving Matheny did was to the ballpark and back. But then extra time in the batting cage produced results.

“We’ve been putting a lot of work in, trying to get my swing right,” said Matheny, son of former Cardinals manager Mike Matheny.

“I’m getting my swing dialed in … Starting to get the feel for it now and figure it out.”

In the past two weeks, Matheny is batting .314 (16 for 51) with seven doubles and his first home run.

“He’s come up and had some big hits for us.” Oliver said. “Nice to see him driving the ball.”

The Sea Dogs (16-32) needed Matheny’s drive because the bullpen gave up a 3-0 lead, including a tying, solo home run by Jared Oliva in the ninth off Durbin Feltman (2-1).

Portland starter Dedgar Jimenez recorded his first scoreless outing, pitching six shutout innings, allowing six hits and one walk, striking out six. As usual, Jimenez’s success depended on commanding his 88-90 mph fastball with quality secondary pitches.

“Just being aggressive with the hitters and staying on top of the strike zone,” Jimenez said. “A lot of fastball and sliders. Later in the game, more change-ups and curves.”

Jimenez began the year with two struggling starts (eight earned runs in 3 2/3 innings), but has been solid since, with six quality starts out of seven.

“Dedgar was outstanding,” Sea Dogs pitching coach Paul Abbott said. “Mixed in a lot more change-ups today. Had a good, quality mix. Executed a good game plan. The change-up is why he was able to hold them scoreless. He had a good depth to it, which he’s been working on.”

Still, Jimenez got another no-decision and is 1-3.

“I feel bad for him. He did a good job,” Oliver said.

“We’ve got to figure out what’s going on with the bullpen. We adjust that, we’re going to give ourselves a good chance to win a lot more games. It’s frustrating. We got the arms on the backside. They’re just not getting the job done.”

Portland got its lead with one in the second inning (Matheny double, Luke Tendler RBI single) and two in the fifth (four straight singles, including RBI from Bobby Dalbec and Josh Tobias).

Jake Cosart relieved in the seventh. With one out, he walked two batters and then left the game with back stiffness. Jordan Weems came in and eventually gave up an unearned run on a passed ball.

Weems allowed an earned run in the eighth on three singles. In the ninth, Oliva’s homer tied it.

In the bottom of the ninth, reliever Matt Eckelman (1-2) recorded two outs, sandwiched around Brett Netzer’s single. But he walked Tobias and Cody Asche to load the bases, bringing up Matheny. Eckelman came with a fastball and Matheny sent it over the right-field wall.

NOTES: As expected, Red Sox infielder Brock Holt was activated and pitcher Darwinzon Hernandez sent back to Portland … The Sea Dogs won 3 of 4 against Altoona, only their fourth series win of the season … Dalbec went 2 for 5 and is batting .240/.825 OPS … The Sea Dogs go on a six-day trip, starting Tuesday in Hartford. They return to Portland on June 4 … The Altoona coaching staff featured some familiar names. Pitching coach Joel Hanrahan was a major league reliever for seven years, including 2013 with Boston, a season cut short by a career-ending elbow injury. Curve hitting coach Jon Nunnally had the same position for the Sea Dogs in 2016.