I apologize for the intrusion but the problem you have (if it is a "problem") isn't reserved for either Mom or Dad. Both get frustrated by the lack of motivation our sons have. I had no long term plans for my son to pursue baseball --- that was his idea. I actually wanted him to hunt and fish with me but HE wanted to play baseball ---- so I'm the one that had to make the major adjustments. Sold my bass boat .... quit turkey hunting. At the same time I let it be known that I'm not going to make big adjustments in my life and spend my money on his baseball unless he make a commitment too ..... we both understood that. Josh was my best friend (still is) and I wanted what was best for him so this wasn't my plan that I laid out for him. It was his plan that I helped him accomplish.
There are a number of ways you can motivate your son. Number one .... you have to talk with him about it. Explain that you're on HIS side. You're helping HIM achieve HIS goals. (not the other way around). Surely we all have enough Zig Ziglar in us to instill some motivation into a 15 year old.
I also found there were enticements to get him motivated. A new pair of batting gloves. A soft toss machine. When I made a sacrifice for him he usually followed by trying harder. I devised games that he and I would play that made practice fun. We live in the country so there were no "teammates". For the most part it was he and I doing or talking baseball.
I suggest you don't push or nudge --- but rather support him. Oh sure I had to bite my tongue from time to time when he slacked up. I never quit supporting him .... and he knew I would support him even when he decided to quit for good .... and I did, 20 years later.
So the first thing you do is you ASK him... "What's your goal in baseball?" If he says "play college baseball" then ask him what his plan is.. Here is where your maturity and parenting has a chance to shine as you explain there "ain't no free lunch"!!!
Good Luck
Fungo
PS: Be sure and explain to him that "A goal without a plan is a dream."